contrib/subtree: document 'push' does not take '--squash'

git subtree push does not support --squash, as previously illustrated in
6ccc71a9 (contrib/subtree: there's no push --squash, 2015-05-07)

Signed-off-by: Danny Lin <danny0838@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.txt

@@ -143,8 +143,8 @@ OPTIONS
 	Specify <message> as the commit message for the merge commit.
 
 
-OPTIONS FOR add, merge, push, pull
-----------------------------------
+OPTIONS FOR add, merge, and pull
+--------------------------------
 --squash::
 	This option is only valid for add, merge, and pull
 	commands.