Improve visp-config.1 man page

Hi Thomas,

First of all, best wishes for this new year.

I have introduced minor changes in visp-config.1 to fix typo and introduce ViSP homepage.

Note that lintian detects an error "manpage not compressed".
visp-config.1 should be compressed with maximum compression using gzip -9

This man page is now introduced in ViSP 2.6.3

Fabien