handbook: Adjust wording of statement in updating/upgrading

The introductory line of 26.8, "Building on non-FreeBSD Hosts" states:
"Nowadays, the FreeBSD can be build on Linux distributions and macOS."
"can be build on" should be "can be built on".
", the FreeBSD can' should be ", FreeBSD can".

Reviewed by: Graham Perrin <grahamperrin@gmail.com>
Reviewed by: Pau Amma <pauamma@gundo.com>, dbaio
Pull Request: https://github.com/freebsd/freebsd-doc/pull/441
Co-authored-by: Graham Perrin <grahamperrin@gmail.com>