FreeBSD Handbook: fix and improve a pair of warnings

Installing Applications: Packages and Ports – Overview of Software
Installation – Warning

Fix the word doubling (use use) that was found by Mohammadreza in the
second warning in db3bba9456.

Whilst here:

• make the first of the two warnings more concise

• reword the second warning, to be consistent with the NAME of pkg-audit(8).

Reported-by: Mohammadreza (GitHub user slash071)
Fixes: db3bba9456 handbook: Wordsmith a fragment about pkg-audit(8)
Pull-request: https://github.com/freebsd/freebsd-doc/pull/216