content/static/html/helpers: lowercase module in directories section

The names in the page type labels in the header are lowercase. Also
lowercase "module" in the label in the directories section to match.

Change-Id: I55e497ced0a8c32994487cb8939107ba795d651a
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/288911
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

content/static/html/helpers/_unit_directories.tmpl

@@ -36,7 +36,7 @@
                   {{- end -}}
                   {{- if .Root -}}
                     <a href="{{.Root.URL}}">{{.Root.Suffix}}</a>
-                    {{if .Root.IsModule}}<span class="UnitHeader-badge">Module</span>{{end}}
+                    {{if .Root.IsModule}}<span class="UnitHeader-badge">module</span>{{end}}
                   </div>
                   <div class="UnitDirectories-mobileSynopsis">{{.Root.Synopsis}}</div>
                 </div>