transient-toggle-level-limit: Use transient-prefix-object

Since this is the only use of `transient-define-suffix' in
"transient.el" itself, it might be looked at for inspiration, so use
`transient-prefix-object', instead of `transient--prefix', which in
this case would be equivalent, but is intended for internal use only.
Hopefully this helps avoiding some confusion.

lisp/transient.el

@@ -2877,7 +2877,7 @@ transient is active."
      (transient--all-levels-p
       (format "Hide suffix %s"
               (propertize
-               (format "levels > %s" (oref transient--prefix level))
+               (format "levels > %s" (oref (transient-prefix-object) level))
                'face 'transient-higher-level)))
      ("Show all suffix levels")))
   :inapt-if (lambda () (= transient-default-level transient--max-level))