programming-guidelines: don't use the term "pre-release increment"

When reading the text, I first thought that it meant "pre-commit" and
"post-commit". But the pre-commit is useless, it can be done at the same
time as the release commit.

Saying "at the time of release" is clearer in my opinion.

https://bugzilla.gnome.org/show_bug.cgi?id=757735