docs: document exploit

doc/EthernautCTF.md

@@ -23,7 +23,7 @@
 | 19  | [AlienCode](../src/EthernautCTF/AlienCodex.sol)            |  ❌  | [AlienCodeExploit](../test/EthernautCTF/AlienCodexExploit.t.sol.txt)    | The challenge requires to use solidity version `^0.5.0` but unfortunately, the minimum version supported by [forge-std](https://github.com/foundry-rs/forge-std) is `0.6.2`. Thus, the solution of this challenge won't be part of this repository.                                                                             |
 | 20  | [Denial](../src/EthernautCTF/Denial.sol)                   |  ✅  | [DenialExploit](../test/EthernautCTF/DenialExploit.t.sol)               | - Always set the amount of gas when using a low-level call. It will prevent the external contract to consume all the gas.<br>- Check the return value of low-level calls, especially when the address is controlled by someone else.                                                                                            |
 | 21  | [Shop](../src/EthernautCTF/Shop.sol)                       |  ✅  | [ShopExploit](../test/EthernautCTF/ShopExploit.t.sol)                   | - When calling an external contract, always check the returned value before using it!<br>- This challenge is very similar to challenge 11.                                                                                                                                                                                      |
-| 22  | Dex                                                        |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |
+| 22  | [Dex](../src/EthernautCTF/Dex.sol)                         |  ✅  | [DexExploit](../test/EthernautCTF/DexExploit.t.sol)                     | The contract uses a division operation to compute the swap amount which can be exploited because of a precision loss. Indeed, Solidity does not support floating points.                                                                                                                                                        |
 | 23  | DexTwo                                                     |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |
 | 24  | PuzzleWallet                                               |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |
 | 25  | Motorbike                                                  |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |