[chore] engine preferences: fix grammar mistakes (#4883)

searx/templates/simple/preferences/engines.html

@@ -6,7 +6,7 @@
   {%- set ns.checked = false -%}
   {%- if categ == DEFAULT_CATEGORY -%}
     <p>
-      {{- _('This tab does not exists in the user interface, but you can search in these engines by its !bangs.') -}}
+      {{- _('This tab does not exist in the user interface, but you can search with these engines via !bangs.') -}}
       {{- ' ' -}}<a href="{{ url_for('info', pagename='search-syntax') }}">&#9432;</a>
     </p>
   {%- endif -%}