add Mastodon info to main index page

In addition to mentioning that IOCCC uses Mastodon
we added a required HTML line that Mastodon
needs to verify our web site.

sitemap.xml

@@ -19686,11 +19686,11 @@
 </url>
 <url>
     <loc>https://www.ioccc.org/index.html</loc>
-    <lastmod>2024-12-29T09:57:30+00:00</lastmod>
+    <lastmod>2024-12-29T11:30:32+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/index.md</loc>
-    <lastmod>2024-12-29T09:57:30+00:00</lastmod>
+    <lastmod>2024-12-29T11:30:11+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/ioccc.css</loc>