12.4R/errata: beautify the open issues area

An issue that is published as an erratum – at a page such as
https://www.freebsd.org/releases/12.4R/errata/
– is inherently important, no need to describe it as such.

By not using a table to present the erratum, we avoid the two rendering
issues that were pictured by emaste at
https://reviews.freebsd.org/D38999#887707

Whilst here:

• add a missing full stop (.)
• one sentence per line.

Approved by: emaste
Fixes: 84b3c465a2 Add OpenSSH updating note to 12.4 errata page
Differential revision: https://reviews.freebsd.org/D39243