docs: fix value boundary

[ChmielewskiKamil]

Isn't the boundary val = val % token.balanceOf(address(this)) missing the maximal value of token.balanceOf(address(this))?

In the Learn how to fuzz like a pro: Intro to AMM's invariants, Justin creates a helper function to create a boundary on the amount.

It looks like this:

function _between(
        uint256 amount,
        uint256 low,
        uint256 high
    ) internal pure returns (uint256) {
        return (low + (amount % (high - low + 1)));
    }

+1 is added to include the maximal value that can be sent.

As explained in this video on the modulo operator by Golan Levin, the modulo operator:

mod	remainder
5%5	0
6%5	1
7%5	2
8%5	3
9%5	4
10%5	0
11%5	1
...

and the cycle repeats

So in our case

mod	remainder
val % token.balanceOf(address(this))	token.balanceOf(address(this)) -1

So we have to add +1 to cover the gap.

[ChmielewskiKamil]

This simple assertion proves it:

function val_boundary(uint256 value) public returns (uint256) {
        uint256 balance = 1000;
        if (value <= balance) {
            value = value % balance;
            return value;
        }
    }

    function test_val_boundary(uint256 value) public {
        assert(val_boundary(value) != 1000);
    }

The value will never be equal to 1000.

[ggrieco-tob]

This is correct, thanks for the fix!