docs(cloud_firestore): fixing example code in aggregation sum

[CaduRomaniello]

Description of what this PR is changing or adding:

Issues fixed by this PR: #43

Risk Level

• [x] No risk
• [ ] Somewhat Risky
• [ ] High risk

Pre-submit checklist

• [x] This PR follows the Google Developer Documentation Style Guidelines
• [x] This PR uses semantic line breaks