internal/frontend: clean up TestReadmeHTML
internal/frontend: clean up TestReadmeHTML

No longer require the exact trailing space.

Change-Id: I6f93fd57cf7e8a32cc43cabe618ccc3ce024b81c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/243619
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite