Documentation: Add --line-number to git-grep synopsis

Commit 7d6cb10 ("grep: Add the option '--line-number'", 2011-03-28)
introduced the --line-number option and added its description to OPTIONS
section, but forgot to update SYNOPSIS.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -12,7 +12,7 @@ SYNOPSIS
 'git grep' [-a | --text] [-I] [-i | --ignore-case] [-w | --word-regexp]
 	   [-v | --invert-match] [-h|-H] [--full-name]
 	   [-E | --extended-regexp] [-G | --basic-regexp]
-	   [-F | --fixed-strings] [-n]
+	   [-F | --fixed-strings] [-n | --line-number]
 	   [-l | --files-with-matches] [-L | --files-without-match]
 	   [(-O | --open-files-in-pager) [<pager>]]
 	   [-z | --null]