Improve env var description

Author: mrcasals

lib/generators/templates/app.json.erb

@@ -39,7 +39,7 @@
       "required": false
     },
     "HEROKU_APP_NAME": {
-      "description": "Needed for review apps",
+      "description": "Needed for review apps and seed data",
       "required": false
     },
     "SECRET_KEY_BASE": {