docs/process: branching-checklist: Remove reference to root

The steps to generate the documentation for the new branch requires
to ssh as root and then su to run with the user xendocs.

The release technician may not (and should not) have access to root.
So update the step to directly ssh as xendocs.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: George Dunlap <george.dunlap@cloud.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>

docs/process/branching-checklist.txt

@@ -25,8 +25,7 @@ ov=4.0
 * make 13:37 <ijc> https://xenbits.xen.org/docs/4.2-testing/ is now live true
 #14:17 <ijc> HOWTO: login to xenbits. become "xendocs" . cd cronjobs . edit
 #            xenbits-docs-all.sh in the obvious way. git commit
-    ssh root@xenbits.xen.org
-    su - xendocs
+    ssh xendocs@xenbits.xen.org
     cd cronjobs
     ed xenbits-docs-all.sh
     /for branch