Update Orphan Slayer diagram

Following feedback from https://phabricator.wikimedia.org/T295926#7565434

old/orphan_slayer/activity_diagram

@@ -3,7 +3,7 @@ title Ingenico Orphan Slayer
 
 (*) --> "orphan_slayer_process_orphans()"
 
---> "$orphan = PendingDatabase::get_oldest()"
+--> "$orphan = OrphanSlayer::get_oldest()"
 
 if "$orphan['contribution_tracking_id']?" then
   -->[yes] "$slayer->rectify($orphan)"