[fix] preferences: change engine table category colspan to 8

* this is a small fix to increase the colspan of the category in engine preferences from 7 to 8, since there was a column added
=> fixing a small fallout from 4731290317

searx/templates/simple/preferences/engines.html

@@ -35,7 +35,7 @@
         {%- if loop.length > 1 -%}
           <tr>{{- '' -}}
             <th class="engine-group" colspan="2">{{- _(group) -}}</th>{{- '' -}}
-            <th class="engine-group" colspan="7">
+            <th class="engine-group" colspan="8">
               {%- if group_bang -%}
                 <span class="bang">{{- group_bang -}}</span>
               {%- endif -%}</th>{{- '' -}}