Documentation: Fix typo in thin-provisioning.txt

Correct spelling "descibes" to "describes" in
Documentation/device-mapper/thin-provisioning.txt

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/Documentation/device-mapper/thin-provisioning.txt b/Documentation/device-mapper/thin-provisioning.txt
index 801d9d1..1ff044d 100644
--- a/Documentation/device-mapper/thin-provisioning.txt
+++ b/Documentation/device-mapper/thin-provisioning.txt
@@ -1,7 +1,7 @@
 Introduction
 ============
 
-This document descibes a collection of device-mapper targets that
+This document describes a collection of device-mapper targets that
 between them implement thin-provisioning and snapshots.
 
 The main highlight of this implementation, compared to the previous