doc: filesystems : Fix typo in Documentations/filesystems

Correct spelling typo in Documentations/filesystems.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/Documentation/filesystems/btrfs.txt b/Documentation/filesystems/btrfs.txt
index b349d57..9dae594 100644
--- a/Documentation/filesystems/btrfs.txt
+++ b/Documentation/filesystems/btrfs.txt
@@ -87,7 +87,7 @@
 
   device=<devicepath>
 	Specify a device during mount so that ioctls on the control device
-	can be avoided.  Especialy useful when trying to mount a multi-device
+	can be avoided.  Especially useful when trying to mount a multi-device
 	setup as root.  May be specified multiple times for multiple devices.
 
   discard