githooks.txt: add missing word

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -86,7 +86,7 @@ This hook is invoked by 'git-commit' right after preparing the
 default log message, and before the editor is started.
 
 It takes one to three parameters.  The first is the name of the file
-that the commit log message.  The second is the source of the commit
+that contains the commit log message.  The second is the source of the commit
 message, and can be: `message` (if a `-m` or `-F` option was
 given); `template` (if a `-t` option was given or the
 configuration option `commit.template` is set); `merge` (if the