static/frontend: added heading level in homepage.

https://screenshot.googleplex.com/6s5RbUYVVzAMDga.png

Change-Id: Idbe2b8e58f8849aad65b96bf8dc901d733cd9253
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/519355
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Suzy Mueller <suzmue@golang.org>
kokoro-CI: kokoro <noreply+kokoro@google.com>

static/frontend/homepage/homepage.tmpl

@@ -63,7 +63,7 @@
 {{define "pre-footer"}}
   <div class="Questions">
     <div class="Questions-content">
-      <div class="Questions-header">Frequently asked questions:</div>
+      <div class="Questions-header" role="heading" aria-level="1">Frequently asked questions:</div>
       <ul>
         <li><a href="https://go.dev/about#adding-a-package">How can I add a package?</a></li>
         <li><a href="https://go.dev/about#removing-a-package">How can I remove a package?</a></li>