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>

Documentation/githooks.txt

@@ -39,15 +39,15 @@ HOOKS
 applypatch-msg
 ~~~~~~~~~~~~~~
 
-This hook is invoked by 'git am' script.  It takes a single
+This hook is invoked by 'git am'.  It takes a single
 parameter, the name of the file that holds the proposed commit
-log message.  Exiting with non-zero status causes
-'git am' to abort before applying the patch.
+log message.  Exiting with a non-zero status causes 'git am' to abort
+before applying the patch.
 
 The hook is allowed to edit the message file in place, and can
 be used to normalize the message into some project standard
-format (if the project has one). It can also be used to refuse
-the commit after inspecting the message file.
+format. It can also be used to refuse the commit after inspecting
+the message file.
 
 The default 'applypatch-msg' hook, when enabled, runs the
 'commit-msg' hook, if the latter is enabled.
@@ -80,10 +80,10 @@ pre-commit
 ~~~~~~~~~~
 
 This hook is invoked by 'git commit', and can be bypassed
-with `--no-verify` option.  It takes no parameter, and is
+with the `--no-verify` option.  It takes no parameters, and is
 invoked before obtaining the proposed commit log message and
-making a commit.  Exiting with non-zero status from this script
-causes the 'git commit' to abort.
+making a commit.  Exiting with a non-zero status from this script
+causes the 'git commit' command to abort before creating a commit.
 
 The default 'pre-commit' hook, when enabled, catches introduction
 of lines with trailing whitespaces and aborts the commit when
@@ -122,15 +122,15 @@ commit-msg
 ~~~~~~~~~~
 
 This hook is invoked by 'git commit', and can be bypassed
-with `--no-verify` option.  It takes a single parameter, the
+with the `--no-verify` option.  It takes a single parameter, the
 name of the file that holds the proposed commit log message.
-Exiting with non-zero status causes the 'git commit' to
+Exiting with a non-zero status causes the 'git commit' to
 abort.
 
-The hook is allowed to edit the message file in place, and can
-be used to normalize the message into some project standard
-format (if the project has one). It can also be used to refuse
-the commit after inspecting the message file.
+The hook is allowed to edit the message file in place, and can be used
+to normalize the message into some project standard format. It
+can also be used to refuse the commit after inspecting the message
+file.
 
 The default 'commit-msg' hook, when enabled, detects duplicate
 "Signed-off-by" lines, and aborts the commit if one is found.
@@ -138,8 +138,8 @@ The default 'commit-msg' hook, when enabled, detects duplicate
 post-commit
 ~~~~~~~~~~~
 
-This hook is invoked by 'git commit'.  It takes no
-parameter, and is invoked after a commit is made.
+This hook is invoked by 'git commit'. It takes no parameters, and is
+invoked after a commit is made.
 
 This hook is meant primarily for notification, and cannot affect
 the outcome of 'git commit'.