message: fix cache factory category
diff: @@ -321,7 +321,7 @@ public:
  class HttpCacheFactory : public Config::TypedFactory {
  public:
  // From UntypedFactory
  - std::string category() const override { return "http_cache_factory"; }
  + std::string category() const override { return "envoy.http.cache"; }
  // Returns an HttpCache that will remain valid indefinitely (at least as long
  // as the calling CacheFilter).
  