content: use a header for the 'other modules' section
content: use a header for the 'other modules' section

The message 'Other modules containing this package' had an italicized
header, which no longer makes sense after the redesign.

Fixes b/137856715

Change-Id: I3b2a8f113667437f7fe909199cda9ecd3f4bd685
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/509280
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

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