bsd-family-tree: Re-center OpenBSD line

Commit 09aeca28dd75 ("BSD family tree: reduce OpenBSD-NetBSD crowding")
moved OpenBSD releases over 2 spaces. Move the line through OpenBSD
releases over as well.

Reviewed by: obrien
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53635

(cherry picked from commit 1821cf482ff128b08bd65807e1f7aad7eb873e0b)