Update crud-introduction.txt

[tomprimozic]

If I understand MongoDB properly, it allows

{
'a': 1, 2
}

and not

{

}

Or maybe I don't understand the text correctly.

[tychoish]

I think the word "nest" is unclear in this situation: I hope the following commit helps clarify things:

9c2efd6

[tychoish]

Thanks so much for your attention and consideration! I look forward to merging fixes for other issues that you may find!