cmd/coordinator: HTML escape another URL
cmd/coordinator: HTML escape another URL

This was missed from CL 179657.

Change-Id: I7e3966c1cd02f347b0f3ce96c13951b86a6a2b9b
Reviewed-on: https://go-review.googlesource.com/c/build/+/179697
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build