[themes] add comma before "but" in new sentence

Closes #739.

searx/templates/oscar/preferences.html

@@ -322,7 +322,7 @@
                     </noscript>
                     <div class="tab-pane{% if loop.first %} active{% endif %} active_if_nojs" id="tab_engine_{{ categ|replace(' ', '_') }}">
                         {% if categ == OTHER_CATEGORY %}
-                            <p>{{_('This tab does not show up for search results but you can search the engines listed here via bangs.')}}</p>
+                            <p>{{_('This tab does not show up for search results, but you can search the engines listed here via bangs.')}}</p>
                         {% endif %}
                         <div class="container-fluid">
                         <fieldset>

searx/templates/simple/preferences.html

@@ -277,7 +277,7 @@
     {% for categ in categories_as_tabs + [OTHER_CATEGORY] %}
     {{ tab_header('enginetab', 'category' + categ, _(categ)) }}
     {% if categ == OTHER_CATEGORY %}
-      <p>{{_('This tab does not show up for search results but you can search the engines listed here via bangs.')}}</p>
+      <p>{{_('This tab does not show up for search results, but you can search the engines listed here via bangs.')}}</p>
     {% endif %}
     <div class="scrollx">
     <table class="striped">