Btrfs: fix typos in comments

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 86c44e9..2185de7 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -76,7 +76,7 @@
 struct btrfs_fs_devices {
 	u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
 
-	/* the device with this id has the most recent coyp of the super */
+	/* the device with this id has the most recent copy of the super */
 	u64 latest_devid;
 	u64 latest_trans;
 	u64 num_devices;