Update "Fork & Edit on GitHub" links to point to precise file (#4898)

This resolves the only remaining issue of #3637 involving the "Fork &
Edit on GitHub" link, which applies to all individual Technique and
Understanding pages. This was always statically pointing to the repo's
new issue URL, which was behavior carried over from the previous XSLT
build system. Now that we're using Eleventy, this is trivial to
implement consistently with the rest of the WAI website.

Fixes #3637.

Author: kfranqueiro

_includes/help-improve.html

@@ -10,7 +10,7 @@ <h2> Help improve this page </h2>
     <div class="button-group">
       <a href="mailto:public-agwg-comments@w3.org?subject=%5BUnderstanding%20and%20Techniques%20Feedback%5D"
         class="button"><span>Email</span></a>
-      <a href="https://github.com/w3c/wcag/issues/" class="button"><span>Fork &amp; Edit on GitHub</span></a>
+      <a href="https://github.com/w3c/wcag/edit/main/{{ page.inputPath | replace_first: "./", "" }}" class="button"><span>Fork &amp; Edit on GitHub</span></a>
       <a href="https://github.com/w3c/wcag/issues/new" class="button"><span>New GitHub Issue</span></a>
     </div>
   </div>