Documentation: Fix typo in thin-provisioning.txt

Correct spelling "descibes" to "describes" in
Documentation/device-mapper/thin-provisioning.txt
Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

Documentation/device-mapper/thin-provisioning.txt

 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