clarify _spendAllowance natspec (#3188)

contracts/token/ERC20/ERC20.sol

@@ -320,7 +320,7 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
     }
 
     /**
-     * @dev Spend `amount` form the allowance of `owner` toward `spender`.
+     * @dev Updates `owner` s allowance for `spender` based on spent `amount`.
      *
      * Does not update the allowance amount in case of infinite allowance.
      * Revert if not enough allowance is available.

contracts/token/ERC777/ERC777.sol

@@ -502,7 +502,7 @@ contract ERC777 is Context, IERC777, IERC20 {
     }
 
     /**
-     * @dev Spend `amount` form the allowance of `owner` toward `spender`.
+     * @dev Updates `owner` s allowance for `spender` based on spent `amount`.
      *
      * Does not update the allowance amount in case of infinite allowance.
      * Revert if not enough allowance is available.