Action Cable: use non-deprecated entrypoint for JS package

Author: georgeclaghorn

actioncable/package.json

@@ -3,7 +3,7 @@
   "version": "7.0.0-alpha2",
   "description": "WebSocket framework for Ruby on Rails.",
   "module": "app/javascript/action_cable/index.js",
-  "main": "app/assets/javascripts/action_cable.js",
+  "main": "app/assets/javascripts/actioncable.js",
   "files": [
     "app/assets/javascripts/*.js",
     "src/*.js"