Use msg.sender in docs

contracts/access/AccessControl.sol

@@ -21,7 +21,7 @@ import "../GSN/Context.sol";
  *
  * ```
  * function foo() public {
- *     require(hasRole(MY_ROLE, _msgSender()));
+ *     require(hasRole(MY_ROLE, msg.sender));
  *     ...
  * }
  * ```