Fix ERC777 hook docs, fixes #2256

contracts/token/ERC777/ERC777.sol

@@ -491,13 +491,13 @@ contract ERC777 is Context, IERC777, IERC20 {
      *
      * Calling conditions:
      *
-     * - when `from` and `to` are both non-zero, ``from``'s `tokenId` will be
-     * transferred to `to`.
-     * - when `from` is zero, `tokenId` will be minted for `to`.
-     * - when `to` is zero, ``from``'s `tokenId` will be burned.
+     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
+     * will be to transferred to `to`.
+     * - when `from` is zero, `amount` tokens will be minted for `to`.
+     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
      * - `from` and `to` are never both zero.
      *
      * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
      */
-    function _beforeTokenTransfer(address operator, address from, address to, uint256 tokenId) internal virtual { }
+    function _beforeTokenTransfer(address operator, address from, address to, uint256 amount) internal virtual { }
 }