stash doc: document short form -p in synopsis

'git stash save' can take -p, the short form of --patch, as an option.
Document this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-stash.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 'git stash' drop [-q|--quiet] [<stash>]
 'git stash' ( pop | apply ) [--index] [-q|--quiet] [<stash>]
 'git stash' branch <branchname> [<stash>]
-'git stash' [save [--patch] [-k|--[no-]keep-index] [-q|--quiet]
+'git stash' [save [-p|--patch] [-k|--[no-]keep-index] [-q|--quiet]
 	     [-u|--include-untracked] [-a|--all] [<message>]]
 'git stash' clear
 'git stash' create [<message>]