beastie theme: en.toml apostrophes and more

Apostrophes are not necessary in phrases such as 'FreeBSD's Handbook'
and 'Documentation's Handbook'.

'Presentation and papers': should be plural, 'Presentations and
papers'.

'Documentation Project Handbook' instead of 'Documentation Handbook'
(still, not as long as the full title 'FreeBSD Documentation Project
Primer for New Contributors').

'Get FreeBSD' instead of 'Download FreeBSD'.

Three other minor changes (Articles, Books, Forum).

Reviewed by: ceri, pauamma
Approved by: carlavilla
Differential revision: https://reviews.freebsd.org/D37503