Fix typo in Proxy.sol

contracts/proxy/Proxy.sol

@@ -53,7 +53,7 @@ abstract contract Proxy {
     /**
      * @dev Delegates the current call to the address returned by `_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 _fallback() internal virtual {
         _beforeFallback();