Fix openapi-tags-alphabetical rule invalid property declaration

The 'properties' key of the 'alphabetical' rule must be an array of
strings instead of a string.

Author: Martin D'Aloia

rules/default.yaml

@@ -45,7 +45,8 @@ rules:
   object: openapi
   description: openapi object should have alphabetical tags
   alphabetical:
-    properties: tags
+    properties:
+      - tags
     keyedBy: name
 - name: reference-no-other-properties
   object: reference