Documentation/git-rebase: fix --no-autostash formatting

All of the other "--option" and "--no-option" pairs in this file are
formatted as separate options.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -417,7 +417,8 @@ If the '--autosquash' option is enabled by default using the
 configuration variable `rebase.autosquash`, this option can be
 used to override and disable this setting.
 
---[no-]autostash::
+--autostash::
+--no-autostash::
 	Automatically create a temporary stash before the operation
 	begins, and apply it after the operation ends.  This means
 	that you can run rebase on a dirty worktree.  However, use