Fixed a typo in a comment. (#1422)

(cherry picked from commit 3341dd88994ee468c6b36a1affa4608dedab7a08)

contracts/introspection/ERC165.sol

@@ -42,7 +42,7 @@ contract ERC165 is IERC165 {
   }
 
   /**
-   * @dev private method for registering an interface
+   * @dev internal method for registering an interface
    */
   function _registerInterface(bytes4 interfaceId)
     internal