Update getting_started.ezdoc

Make example code dialyzer approved.

doc/src/guide/getting_started.ezdoc

@@ -121,7 +121,7 @@ start(_Type, _Args) ->
 	Dispatch = cowboy_router:compile([
 		{'_', [{"/", hello_handler, []}]}
 	]),
-	cowboy:start_http(my_http_listener, 100, [{port, 8080}],
+	{ok, _} = cowboy:start_http(my_http_listener, 100, [{port, 8080}],
 		[{env, [{dispatch, Dispatch}]}]
 	),
 	hello_erlang_sup:start_link().