Made _burn internal instead of private (#1908)

contracts/token/ERC777/ERC777.sol

@@ -377,7 +377,7 @@ contract ERC777 is Context, IERC777, IERC20 {
         bytes memory data,
         bytes memory operatorData
     )
-        private
+        internal
     {
         require(from != address(0), "ERC777: burn from the zero address");