push: add '--delete' flag to synopsis

The delete flag is not mentioned in the synopsis of `git-push`.
Add the flag to make it more discoverable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-push.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git push' [--all | --mirror | --tags] [--follow-tags] [--atomic] [-n | --dry-run] [--receive-pack=<git-receive-pack>]
-	   [--repo=<repository>] [-f | --force] [--prune] [-v | --verbose]
+	   [--repo=<repository>] [-f | --force] [--delete] [--prune] [-v | --verbose]
 	   [-u | --set-upstream]
 	   [--[no-]signed|--sign=(true|false|if-asked)]
 	   [--force-with-lease[=<refname>[:<expect>]]]