pull: Fix FreeBSD SPDX identifier (to BSD-2-Clause-FreeBSD)

pull: Fix FreeBSD SPDX identifier (to BSD-2-Clause-FreeBSD)

Based on Kate's list [1].  Looking more closely, the FSF's FreeBSD
text [2] includes the "The views and conclusions..." paragraph that is
unique to the BSD-2-Clause-FreeBSD [3].  The FSF's FreeBSD page [2]
also links [4].

[1]: https://github.com/spdx/license-list-XML/pull/453#issuecomment-338411910
[2]: https://directory.fsf.org/wiki?title=License:FreeBSD
[3]: https://github.com/spdx/license-list-XML/blob/e5da40e25becb0aa7626d3f62649d2387284a623/src/BSD-2-Clause-FreeBSD.xml#L38-L40
[4]: https://www.freebsd.org/copyright/freebsd-license.html