Release errata errata (punctuation) and markup

Late follow-up to
d343022941 12.4R/errata: beautify the open issues area

• that included a correction to punctuation in the first paragraph of the introduction.

For 13.0R, 13.1R, and 13.2R:

• add the missing full stop

• improve markup (line breaks).

Pull-request: https://github.com/freebsd/freebsd-doc/pull/214