githooks.txt: minor improvements to the grammar & phrasing
Change:
* Sentences that needed "the" or "a" to either add those or change them
so they don't need them.
* The little tangent about "You can use this to do X (if your project
wants to do X)" can just be shortened to "if you want to do X".
* s/parameter/parameters/ when the plural made more sense.
Most of this goes all the way back to the initial introduction of
hooks.txt in
6d35cc76 (Document hooks., 2005-09-02).
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>