Documentation: Fix a typo in mailbox.txt

This patch fix a typo in Documentation/mailbox.txt
"bool async" is defined in struct demo_client.
Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/mailbox.txt

@@ -53,7 +53,7 @@ static void message_from_remote(struct mbox_client *cl, void *mssg)
 {
 	struct demo_client *dc = container_of(mbox_client,
 						struct demo_client, cl);
-	if (dc->aysnc) {
+	if (dc->async) {
 		if (is_an_ack(mssg)) {
 			/* An ACK to our last sample sent */
 			return; /* Or do something else here */