GNU Social -> Mastodon link in header

Over the past year, my GNU Social timeline has gone almost completely
silent; it seems that many people have moved to Mastodon and maybe those
instances have stopped federating.

Further, GNU Social development has been stalled for a long time.

So this seems like an inevitable decision to give Mastodon a try.  I'll
start by following people and will post both on here and GNU Social
initially.  See https://social.mikegerwitz.com.

src/header.tpl.htm

@@ -24,7 +24,7 @@
           <li><a href="/talks">Talks</a></li>
           <li><a href="/projects">Projects</a></li>
           <li><a href="/papers">Papers</a></li>
-          <li><a href="//social.mikegerwitz.com/" title="My GNU Social Instance">Social</a></li>
+          <li><a href="//mastodon.mikegerwitz.com/" title="My Mastodon Instance">Social</a></li>
         </ul>
       </nav>
     </header>