frontend: set service to play

Fixes golang/go#21645

Change-Id: I07ca2e63cbaffb96ace0dccf7ee1c112ca5a5417
Reviewed-on: https://go-review.googlesource.com/85036
Reviewed-by: Andrew Bonventre <andybons@golang.org>

frontend/app.yaml

diff --git a/frontend/app.yaml b/frontend/app.yaml
index d3584df..55df140 100644
--- a/frontend/app.yaml
+++ b/frontend/app.yaml
@@ -1,3 +1,3 @@
-service: play-flex
+service: play
 runtime: custom
 env: flex