message: fix(aio): add a required comma in firebase.json
diff: {"type": 301, "source": "/guide/metadata", "destination": "/guide/aot-compiler"},
  // ngmodule.md renamed to ngmodules.md
  - {"type": 301, "source": "/guide/ngmodule", "destination": "/guide/ngmodules"}
  + {"type": 301, "source": "/guide/ngmodule", "destination": "/guide/ngmodules"},
  // service-worker-getstart.md, service-worker-comm.md, service-worker-configref.md
  {"type": 301, "source": "/guide/service-worker-getstart", "destination": "/guide/service-worker-getting-started"},
  