mod_muc_mam: Improve wording of enable setting

Suggested by jstein in the chat

This option label is used by XMPP clients to explain what the option does.
a) The user should know where the data is archived.
b) The user needs a statement that can be enabled/disabled by the variable. A question would have the wrong logic here.

plugins/mod_muc_mam.lua

@@ -90,7 +90,7 @@ if not log_all_rooms then
 		{
 			name = muc_form_enable,
 			type = "boolean",
-			label = "Enable archiving?",
+			label = "Archive chat on server",
 			value = archiving_enabled(room),
 		}
 		);