fixes overwrite mark prompt (cmss13-devs#4432)

# About the pull request
As title says. Allows staff to press the X button without it overwriting
the mark.
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# Explain why it's good for the game
# Testing Photographs and Procedure
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# Changelog
:cl:
fix: Fixes the overwrite mark prompt on admin tickets not respecting
closing the confirmation prompt.
/:cl:

code/modules/admin/verbs/adminhelp.dm

@@ -530,11 +530,11 @@ GLOBAL_DATUM_INIT(ahelp_tickets, /datum/admin_help_tickets, new)
 			return
 		to_chat(usr, SPAN_WARNING("This ticket has already been marked by [marked_admin]."))
 		var/unmark_option = tgui_alert(usr, "This message has been marked by [marked_admin]. Do you want to override?", "Marked Ticket", list("Overwrite Mark", "Unmark", "Cancel"))
-		if(unmark_option == "Cancel")
-			return
 		if(unmark_option == "Unmark")
 			unmark_ticket()
 			return
+		if(unmark_option != "Overwrite Mark")
+			return
 
 	var/key_name = key_name_admin(usr)
 	AddInteraction("Marked by [key_name].", player_message = "Ticket marked!")