Fix merge issue in CHANGELOG.md

CHANGELOG.md

@@ -1,12 +1,12 @@
 # Changelog
 
-## 4.0.0 (2021-03-23)
+## Unreleased
 
  * `IERC20Metadata`: add a new extended interface that includes the optional `name()`, `symbol()` and `decimals()` functions. ([#2561](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2561))
  * `ERC777`: make reception acquirement optional in `_mint`. ([#2552](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2552))
  * `ERC20Permit`: add a `_useNonce` to enable further usage of ERC712 signatures. ([#2565](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2565))
 
-## Unreleased
+## 4.0.0 (2021-03-23)
 
  * Now targeting the 0.8.x line of Solidity compilers. For 0.6.x (resp 0.7.x) support, use version 3.4.0 (resp 3.4.0-solc-0.7) of OpenZeppelin.
  * `Context`: making `_msgData` return `bytes calldata` instead of `bytes memory` ([#2492](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2492))