ui: fix grammar mistakes (#570)

packages/ui-default/templates/ranking.html

@@ -8,7 +8,7 @@
       <div class="section__body no-padding">
         <div data-fragment-id="ranking">
           {% if not udocs.length %}
-          {{ nothing.render('Sorry, there is no user in this domain.') }}
+          {{ nothing.render('Sorry, there are no users in this domain.') }}
           {% else %}
           <table class="data-table">
             <colgroup>