Fix for answering marked tickets. (#5897)

# About the pull request

Followup to #5872. Should have searched through all the files instead of
only one to begin withal.

# Explain why it's good for the game

Is fix.

# Changelog

:cl:
admin: Actually fixed the last case of being warned against interacting
with ticket already marked by you.
/:cl:

code/modules/admin/verbs/adminpm.dm

@@ -60,7 +60,7 @@
 
 	var/message_prompt = "Message:"
 
-	if((AH?.opening_responders && length(AH.ticket_interactions) == 1 ) || ((AH?.marked_admin && AH?.marked_admin != usr.key) && length(AH.ticket_interactions) == 2))
+	if((AH?.opening_responders && length(AH.ticket_interactions) == 1 ) || ((AH?.marked_admin && AH?.marked_admin != usr.ckey) && length(AH.ticket_interactions) == 2))
 		SEND_SOUND(src, sound('sound/machines/buzz-sigh.ogg', volume=30))
 		message_prompt += "\n\n**This ticket is already being responded to by: [length(AH.opening_responders) ? english_list(AH.opening_responders) : AH.marked_admin]**"