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 <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/mailbox.txt b/Documentation/mailbox.txt
index 60f43ff..1092ad9 100644
--- a/Documentation/mailbox.txt
+++ b/Documentation/mailbox.txt
@@ -53,7 +53,7 @@
 {
 	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 */