Mk/Scripts/desktop-categories.sh: fix whitespace issue, alphabetize

A copy paste error from when the patch set was reworked to name the
category "filesystems" instead of "fs."

Reported by: danfe
Fixes: 6e2da9672f79f44048d597f0f61e4646cdeade9d
Pull Request: https://github.com/freebsd/freebsd-ports/pull/302
PR: 281988