cmd/releasebot: relax requirements for release documentation check
cmd/releasebot: relax requirements for release documentation check

The current behavior is very strict (one newline after the <p> tag
and no indentation before the release name). Relax these requirements
to only check for "<release name> (released ".

This makes it work for major releases in addition to minor releases.

Change-Id: I541a7d5959f0e00b8a8b471e9ce8440c6253bd3a
Reviewed-on: https://go-review.googlesource.com/c/build/+/193057
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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