btrfs: qgroup: fix a typo in a comment

Add a missing 'r'.  s/qgoup/qgroup/ . Codespell does not catch that for
some reason.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>