Mention pass by reference in episode 3

[katrinabrock]

If this pull request addresses an open issue on the repository, please add 'Closes #NN' below, where NN is the issue number.

Closes #370

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

As requested in #370 , this change warns users about of pass-by-reference when they first learn about the concept of assignment.

In order avoid introducing and fully explaining the concepts of objects, pass-by-reference, and mutable/immutable (as @alee does beautifully in this comment), I use the term "links inside [the object]" to describe mutable objects with mutable references. Hopefully this describes the behavior without saying anything technically incorrect or getting too in-the-weeds for Episode 3. Maybe some of this more comprehensive explanation could be added to Episode 11, since the concept is introduced there but not thoroughly explained.

If any relevant discussions have taken place elsewhere, please provide links to these.

I used #499 as a basis and kept the example and structure, but reworded completely.

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