update docs/firestore/documents.md

[obumnwabude]

I had this error that doc does not exist on payload anytime I access DocumentData from snapshotChanges().
However, treating payload as DocumentSnapshot (using the data() method on it directly),
rather than treating it as DocumentChange, solved the problem.

So that's why I feel that the type of payload is DocumentSnapshot and not DocumentChange

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Description

Code sample

[jamesdaniels]

Thanks for the fix!