cli: document usage of "subscribe" command

Support for "subscribe" command was added years ago in the commit
453b87f631 ("cli: add support for subscribing to objects"). Document
its usage.

Cc: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

cli.c

@@ -549,6 +549,7 @@ static int usage(const char *prog)
 		"Commands:\n"
 		" - list [<path>]			List objects\n"
 		" - call <path> <method> [<message>]	Call an object method\n"
+		" - subscribe <path> [<path>...]	Subscribe to object(s) notifications\n"
 		" - listen [<path>...]			Listen for events\n"
 		" - send <type> [<message>]		Send an event\n"
 		" - wait_for <object> [<object>...]	Wait for multiple objects to appear on ubus\n"