[BUGFIX] Fix for rule prefetch on /search

In f48218e we refactored how rules
stored labels/annotations but when we updated this prefetch entry we
accidentally changed it so that is no longer a proper tuple. This needs
to be a tuple so that it does not try to treat each character as an
entry.

promgen/views.py

@@ -1134,7 +1134,7 @@ def get(self, request):
             "rule_list": {
                 "field": ("name__icontains", "clause__icontains"),
                 "model": models.Rule,
-                "prefetch": ("content_object"),
+                "prefetch": ("content_object",),
                 "query": ("search",),
             },
             "service_list": {