Add missing pub to storage field in testing internals (#2597)

## Checklist

<!-- Make sure all of these are complete -->

- [x] Linked relevant issue
- [x] Updated relevant documentation
- [x] Added relevant tests
- [x] Performed self-review of the code
- [ ] Added changes to `CHANGELOG.md` <-- **Is this necessary for docs
change?**

docs/listings/snforge_overview/crates/testing_contract_internals/src/basic_example.cairo

@@ -5,7 +5,7 @@ trait IContract<TContractState> {}
 pub mod Contract {
  #[storage]
  pub struct Storage {
- balance: felt252,
+ pub balance: felt252,
  }
 
  #[generate_trait]