13.2R notes: reword 'now has a -r option'

If the word "a" precedes a phrase that begins with a hyphen or dash
followed by a vowel sound, a speaker might:

• mispronounce the word as "an"
• omit the hyphen/dash part of the phrase.

In cases such as this, consider alternative wording.

Approved by: karels
Differential revision: https://reviews.freebsd.org/D39147