Re-label the quick-add widget placeholder 'tag' to 'watch group'

changedetectionio/templates/watch-overview.html

@@ -10,7 +10,7 @@
         <fieldset>
             <legend>Add a new change detection watch</legend>
                 {{ render_simple_field(form.url, placeholder="https://...", required=true) }}
-                {{ render_simple_field(form.tag, value=active_tag if active_tag else '', placeholder="tag") }}
+                {{ render_simple_field(form.tag, value=active_tag if active_tag else '', placeholder="watch group") }}
             <button type="submit" class="pure-button pure-button-primary">Watch</button>
         </fieldset>
         <!-- add extra stuff, like do a http POST and send headers -->