gui: HTML accessibility updates

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4258
LGTM: calmh

gui/default/syncthing/development/logbar.html

@@ -1,5 +1,5 @@
 <div class="dev-top-bar" id="dev-top-bar" style="display: none">
-    <link href="assets/css/dev.css" rel="stylesheet">
+    <link href="assets/css/dev.css" rel="stylesheet"/>
     <div class="row">
         <div class="col-xs-4"><b>DEV</b></div>
         <div id="log" class="col-xs-8">