Replace println! with assert! if possible in DataFusion examples

[comphead]

we can use assert! in DataFusion examples to let the user know the expected output

Originally posted by @comphead in #11088 (comment)

[alamb]

I added this as a subtask of #11230

I would like to propose we migrate some of the smaller examples over from datafusion-examples and into the user guide (to make them more discoverable)

Perhaps we can update them to use assert at the same time, as I found I had to do in #11217

[Nishi46]

take

[ShreyasSN]

take