Cast collectionData to avoid TS errors

[ahmetkizilay]

Using the current code example, I am getting an error message Typescript error message like:

Type 'Observable<(DocumentData | (DocumentData & { .... }))[]>' is not assignable to type 'Observable<Item[]>'.

This change also makes this code example more consistent with the current example in firestore.md

Checklist

• Issue number for this PR: none.
• Docs included?: yes.
• Test units included?: no
• In a clean directory, yarn install, yarn test run successfully? yes.

Description

Code sample