Update public comments email subject (#3574)

Public comments email subject has not been updated with WCAG 2.2.

Current version: `WCAG 2.1 public comment`
Updated version: `WCAG 2.2 public comment`


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<a
href="https://pr-preview.s3.amazonaws.com/remibetin/wcag/pull/3574.html"
title="Last updated on Nov 29, 2023, 9:05 AM UTC (913f695)">Preview</a>
| <a
href="https://pr-preview.s3.amazonaws.com/w3c/wcag/3574/0c1ca5f...remibetin:913f695.html"
title="Last updated on Nov 29, 2023, 9:05 AM UTC (913f695)">Diff</a>

guidelines/index.html

@@ -23,7 +23,7 @@
             Although the proposed Success Criteria in this document reference issues tracking
             discussion, the Working Group requests that public comments be filed as new issues,
             one issue per discrete comment. It is free to create a GitHub account to file issues.
-            If filing issues in GitHub is not feasible, send email to <a href="mailto:public-agwg-comments@w3.org?subject=WCAG%202.1%20public%20comment">public-agwg-comments@w3.org</a> (<a href="https://lists.w3.org/Archives/Public/public-agwg-comments/">comment archive</a>).</p>
+            If filing issues in GitHub is not feasible, send email to <a href="mailto:public-agwg-comments@w3.org?subject=WCAG%202.2%20public%20comment">public-agwg-comments@w3.org</a> (<a href="https://lists.w3.org/Archives/Public/public-agwg-comments/">comment archive</a>).</p>
         </section>
         <section class="informative introductory" id="intro">
 			<h2>Introduction</h2>