Use -F instead of -f in tail(1) examples

There is an example in tail(1) manual page explaining how to use tail(1) to
track the contents of /var/log/messages. The example uses the -f flag to
follow the file. The problem with the -f flag is that it cannot handle the
situation where /var/log/messages is rotated. Hence, use -F instead in the
example.

Reviewed by: asomers
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D24157