Change "Self::BlockNumber" to "BlockNumberFor<Self>" (#2104)

change 
```rust
Self::BlockNumber
```

```rust
BlockNumberFor<Self>
```

Co-authored-by: Kian Paimani <[email protected]>

content/md/en/docs/reference/how-to-guides/pallet-design/incorporate-randomness.md

@@ -30,7 +30,7 @@ The guide also illustrates how to add entropy to the randomness value by assigni
    ```rust
    #[pallet::config]
    pub trait frame_system::Config {
-   	type MyRandomness: Randomness<Self::Hash, Self::BlockNumber>;
+   	type MyRandomness: Randomness<Self::Hash, BlockNumberFor<Self>>;
    }
    ```