stash: replace "git stash save" with "git stash push" in the documentation

stash: replace "git stash save" with "git stash push" in the documentation

"git stash push" is the newer interface for creating a stash.  While we
are still keeping "git stash save" around for the time being, it's better
to point new users of "git stash" to the more modern (and more feature
rich) interface, instead of teaching them the older version that we
might want to phase out in the future.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>