Fix broken example of simpledemo.lua to work with "simple demo layout"

[hansonchar]

Motivation for this change

I tried out the example at 36.2.1 The Hello World of Graph Drawing but it didn't compile:

\documentclass{standalone}
\usepackage{fp,pgf,tikz,xcolor}
\usetikzlibrary{graphs,graphdrawing} \usegdlibrary{simpledemo}
\begin{document}
\tikz [very simple demo layout]
  \graph { f -> c -> e -> a -> {b -> {c, d, f}, e -> b}};
\end{document}

It seems to me the culprit is the implementation in SimpleDemo.lua#L25-L39 differs from that in the documentation.

Checklist

• [x] Code changes are licensed under GPLv2 + LPPLv1.3c
• [x] Documentation changes are licensed under FDLv1.2

[hansonchar]

~~The generated manual in pdf can be downloaded at here. I've also tested the changes locally and the example above now works.~~

I've subsequently forced push a better fix. Waiting for Github workflow to complete.