route(8): clarify -prefixlen description

Try to reword -prefixlen section to more clearly and accurately describe how
the -prefixlen modifier works.

While here, fix a word that igor considered a typo: aggregatable addresses is a
valid technical term per RFC-2374, however, it was superseded by the term
"aggregator" in RFC-3587.

MFC after: 1 week
Reviewed by: 0mp, crees
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D10087