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
<h2>{{if .OtherModules}}Versions in this module{{else}}Versions{{end}}</h2>
{{template "module_list" .ThisModule}}
{{if .OtherModules}}
- <em>Other modules containing this package:</em>
+ <h2>Other modules containing this package</h2>
{{template "module_list" .OtherModules}}
{{end}}
{{end}}
