doc: fix architecture.png

Update line colors to match and add missing arrow from frontend to
frontend task queue.

Change-Id: I988fcc8a7ed012980c4cbccf0dc58af23ffc8922
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/237940
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/doc/architecture.png b/doc/architecture.png
index 4afcb50..618d91b 100644
--- a/doc/architecture.png
+++ b/doc/architecture.png
Binary files differ