appdata: use canonical docs.kde.org help location

org.kde.kcron.metainfo.xml

@@ -90,7 +90,7 @@
   </description>
   <url type="homepage">https://www.kde.org</url>
   <url type="bugtracker">https://bugs.kde.org/enter_bug.cgi?format=guided&amp;product=kcron</url>
-  <url type="help">https://docs.kde.org/trunk5/en/kdeadmin/kcontrol5/kcron/index.html</url>
+  <url type="help">https://docs.kde.org/?application=kcontrol5%2Fkcron</url>
   <screenshots>
     <screenshot type="default">
       <image type="source">https://cdn.kde.org/screenshots/kcron/kcron.png</image>