intro.9: minor changes

A correction: 'and' -> 'an'. Plus, several tweaks for brevity or
clarity.

Reviewed by: mhorne
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/813

(cherry picked from commit 712fd5ac9181b544d8ffcf88ab6313233c8ddaed)