Add an example to pw.8 about how to add an existing user to a group.

Add an example to pw.8 about how to add an existing user to a group.

Instead of using pw to modify group membership, users often edit
/etc/group by hand, which is discouraged.  Provide an example of
adding a user to the wheel group, which is a common use case.
I'm using a different user here as in the previous example as that
deleted the user (although the examples don't necessarily have to
be followed in order).

Reviewed by:	rgrimes,0mp
Approved by:	0mp
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D19123