Provide realistic code example

[bob-lee]

Current code example does afDb.object('items').snapshotChanges() which would give a key value of 'items' followed by all items.

Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, npm install, npm run build, and npm test run successfully? (yes/no; required)

Description

Code sample

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[bob-lee]

I signed CLA.

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[orahul1]

thank you man....

[norwebcl]

I signed CLA too, i have a few questions about, firestore docs. Regards!