MAINTAINERS: btrfs: fix git repo URL

The git repo listed for btrfs hasn't been updated in over a year.
List the current one instead.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>