Improve documentation in UUPSUpgradeable (#3606)

contracts/proxy/utils/UUPSUpgradeable.sol

@@ -46,7 +46,7 @@ abstract contract UUPSUpgradeable is IERC1822Proxiable, ERC1967Upgrade {
 
     /**
      * @dev Implementation of the ERC1822 {proxiableUUID} function. This returns the storage slot used by the
-     * implementation. It is used to validate that the this implementation remains valid after an upgrade.
+     * implementation. It is used to validate the implementation's compatibility when performing an upgrade.
      *
      * IMPORTANT: A proxy pointing at a proxiable contract should not be considered proxiable itself, because this risks
      * bricking a proxy that upgrades to it, by delegating to itself until out of gas. Thus it is critical that this