Fix unclosed <input> tag in buildslave.html

This is XHTML; you need the self-closing slash!

buildbot/status/web/templates/buildslave.html

@@ -41,7 +41,7 @@ <h2>Slave information</h2>
 {% elif control %}
   {% if not slave.getGraceFul %}
     <form method="POST" action="{{ shutdown_url }}">
-      <input type="submit" value="Gracefully Shutdown">
+      <input type="submit" value="Gracefully Shutdown"/>
     </form>
   {% else %}
     <p>Gracefully shutting down...</p>