Use stable serverfault.com link for square bracket comparison link

[ethagnawl]

The linked serverfault.com answer is subject to change or be deleted and, if the current answer is to remain the reference, it should be made stable via archive.org.

I'll submit a PR but here's the archive.org capture I just created: https://web.archive.org/web/20240205165239/https://serverfault.com/questions/52034/what-is-the-difference-between-double-and-single-square-brackets-in-bash/52050#52050