freebsd-tips: freebsd-update fetch, then install

In end of life (EOL) warning cases: freebsd-update fetch install (the
two commands, combined) may fetch and patch, but not install.

Instead: run the two consecutively. Consistent with installation
information in /releases/ areas and with accepted
https://reviews.freebsd.org/D42722

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1096