Use versionString() in about modal (fixes #1663)

gui/index.html

@@ -945,7 +945,7 @@
   <!-- About modal -->
 
   <modal id="about" large="yes" close="yes" status="info" title="{{'About' | translate}}">
-    <h1 class="text-center"><img alt="Syncthing" title="Syncthing" src="assets/img/logo-horizontal.svg" style="vertical-align: -16px" height="100" width="366"/><br/><small>{{version}}</small></h1>
+    <h1 class="text-center"><img alt="Syncthing" title="Syncthing" src="assets/img/logo-horizontal.svg" style="vertical-align: -16px" height="100" width="366"/><br/><small>{{versionString()}}</small></h1>
     <hr/>
 
     <p translate>Copyright &copy; 2015 the following Contributors:</p>