unix.7: Correct example

unix.7: Correct example

The example is misleading. It is not a good idea to unlink an
existing socket because we might try to start the server multiple
times. In this case it is preferable to receive an error.

We could add code that removes the socket when the server process
is killed but that would stretch the example too far.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>