Documentation: filesystems: btrfs: Fixed typos and whitespace

I am a high school student trying to become familiar with
Linux kernel development. The btrfs documentation in
Documentation/filesystems had a few typos and errors in
whitespace. This patch corrects both of these.

Signed-off-by: Daniel Grimshaw <grimshaw@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>