Fix wrong PR `CHANGELOG` link (#3957)

CHANGELOG.md

@@ -9,7 +9,7 @@
  * `Math`: optimize `log256` rounding check. ([#3745](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3745))
  * `Strings`: add `equal` method. ([#3774](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3774))
  * `Strings`: add `toString` method for signed integers. ([#3773](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3773))
- * `MerkleProof`: optimize by using unchecked arithmetic. ([#3745](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3745))
+ * `MerkleProof`: optimize by using unchecked arithmetic. ([#3869](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3869))
  * `EnumerableMap`: add a `keys()` function that returns an array containing all the keys. ([#3920](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3920))
 
 ### Deprecations