git-tag.txt: <commit> for --contains is optional

This goes far back to e84fb2f (branch --contains: default to HEAD -
2008-07-08) where the same parsing code is shared with
builtin/tag.c. git-branch.txt correctly states that <commit> for
--contains is optional while git-tag.txt does not. Correct it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-tag.txt

@@ -103,8 +103,9 @@ OPTIONS
 +
 This option is only applicable when listing tags without annotation lines.
 
---contains <commit>::
-	Only list tags which contain the specified commit.
+--contains [<commit>]::
+	Only list tags which contain the specified commit (HEAD if not
+	specified).
 
 --points-at <object>::
 	Only list tags of the given object.