books/handbook/basics: Use less confusing whatis(1) example.

If, as suggested by the Basics chapter in the handbook, one does:

% cd /usr/bin
% man -f * | more

the result is a number of warnings due to filenames like c++ and
flex++ being treated as expressions.

Two ways to handle this:

1. Explain that one should use "man -f -- *" and why;
2. Avoid the problem and use "cd /usr/sbin" instead.

Option 1 is more appropriate for this stage of the handbook

Approved by: blackend (mentor)