Fix exmaple spec

astool/example_custom_spec.jsonld

@@ -1,7 +1,7 @@
 {
   "@context": [
     {
-      "as": "http://www.w3.org/TR/activitystreams-vocabulary",
+      "as": "https://www.w3.org/ns/activitystreams",
       "owl": "http://www.w3.org/2002/07/owl#",
       "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
       "rdfs": "http://www.w3.org/2000/01/rdf-schema#",