Fix visibility in ERC721._checkOnERC721Received documentation (#4386)

contracts/token/ERC721/ERC721.sol

@@ -413,7 +413,7 @@ abstract contract ERC721 is Context, ERC165, IERC721, IERC721Metadata, IERC721Er
     }
 
     /**
-     * @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
+     * @dev Private function to invoke {IERC721Receiver-onERC721Received} on a target address.
      * The call is not executed if the target address is not a contract.
      *
      * @param from address representing the previous owner of the given token ID