Update security_alert_overview.xml

Accidentally reverted a commit from a PR. Fixed.

Author: derkkila

github_app_for_splunk/default/data/ui/views/security_alert_overview.xml

@@ -243,6 +243,9 @@
         <option name="rowNumbers">false</option>
         <option name="totalsRow">false</option>
         <option name="wrap">true</option>
+        <format type="color" field="severity">
+          <colorPalette type="map">{"critical":#DC4E41,"high":#F1813F,"moderate":#F8BE34}</colorPalette>
+        </format>
       </table>
     </panel>
   </row>