fix the demo script, so it doesn't matter where guile is installed

Not everyone has guile installed in /usr/local/bin, this fix uses env to find guile in PATH.

I can't put it all into one shebangs line, since most systems don't split arguments for shebangs and lookup a file named 'guile\ -L\ ./module/' instead. So #! /usr/bin/env guile -L ... is not an option.
The exit in line-3 is required to avoid the shell-script from further executing, since it doesn't stop at !#.
Guile skips everything between #! and !#.

Some thread I found with google, discussing how to run guile scripts:
https://www.sourceware.org/ml/guile/1997-11/msg00035.html