Example hello world generator shouldn't iterate trees

[chsienki]

Our sample hello world generator iterates over the syntax trees. We explicitly tell people not to do this, and use a syntax receiver instead (and want to add an analyzer in the future too).

We should update the sample to not iterate the trees but do something else instead.

[sharwell]

Iterating over syntax trees is only a problem if you are iterating over the SyntaxNode root of the tree. This generator does not call GetRoot[Async|Synchronously] so there is no need to change it.