internal/godoc/dochtml/internal/render: change docDataHTML h3 to h4

The h3 in docDataHTML is changed to an h4, since the Overview heading
was changed from an h2 to an h3.

Change-Id: Ie2b9c6caf25db147f4f1b774a0926a5b874466ae
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260803
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>
diff --git a/internal/godoc/dochtml/internal/render/render.go b/internal/godoc/dochtml/internal/render/render.go
index 312bee8..beed22b 100644
--- a/internal/godoc/dochtml/internal/render/render.go
+++ b/internal/godoc/dochtml/internal/render/render.go
@@ -72,9 +72,9 @@
 var docDataTmpl = template.Must(template.New("").Parse(`
 {{- range .Elements -}}
   {{- if .IsHeading -}}
-    <h3 id="{{.ID}}">{{.Title}}
+    <h4 id="{{.ID}}">{{.Title}}
     {{- if not $.DisablePermalinks}}<a href="#{{.ID}}">¶</a>{{end -}}
-    </h3>
+    </h4>
   {{else if .IsPreformat -}}
     <pre>{{.Body}}</pre>
   {{- else -}}