Fix typo (#3058)

contracts/proxy/Proxy.sol

@@ -17,7 +17,7 @@ abstract contract Proxy {
     /**
      * @dev Delegates the current call to `implementation`.
      *
-     * This function does not return to its internall call site, it will return directly to the external caller.
+     * This function does not return to its internal call site, it will return directly to the external caller.
      */
     function _delegate(address implementation) internal virtual {
         assembly {