Replace readme example with one that fails on all profiles

[Liam-DeVoe]

The current readme example fails only under the test profile:

use hegel::generators as gs;
use hegel::TestCase;

#[hegel::test]
fn test_addition_commutative(tc: TestCase) {
    let x = tc.draw(gs::integers::<i32>());
    let y = tc.draw(gs::integers::<i32>());
    assert_eq!(x + y, y + x);
}

We should replace it with one that fails everywhere.

See #132 (comment).

[DRMacIver]

It's maybe worth noting that this was originally intended to be a passing test and we only found out later that it was failing. We could just replace it with a passing test.

[Liam-DeVoe]

I think it's cute that it fails. Hypothesis's readme has two examples, one which fails, and it would be cute if they were the same example so we can show off both the syntax and shrinking. https://github.com/HypothesisWorks/hypothesis

[DRMacIver]

I don't like that the hypothesis example relies on an undefined symbol. But we could always define my_sort