in Toolkit sbt example, use flat single-project style

[SethTisue]

a language newcomer on Discord was bitten by this today

note that the examples weren't just unnecessarily complicated, they also used file("example") instead of file("."), so the user ended up having both a root and an example project, and they didn't understand that they needed to put their sources in example/src, because how on earth would they know that?

we can avoid issues like that by simply using the flat style

[adpi2]

I am not convince about that because I think we should slowly discourage people from using the sbt flat style:

• the difference between flat, and ThisBuild, and Global and project.in(file(".")) is non-obvious for beginners
• for single module project we should better use Scala CLI whose syntax is as flat as it can be

However I am okay to use file(".") instead of file("example"). It seems that's what we do in scala/scala3.g8 template.

[SethTisue]

okay, I'll make the change