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: Masanari Iida <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/mailbox.txt

···
 {
 	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 */