docs(firestore): fix typo in FieldValue example

[vonovak]

Description

straightforward, thank you

Related issues

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

---

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[CLAassistant]

All committers have signed the CLA.

[mikehardy]

Ah, the perils of code in comments where no compiler checks it :-). Thanks @vonovak

[codecov]

Codecov Report

Merging #5000 (54951a4) into master (f3b955c) will decrease coverage by 3.63%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5000      +/-   ##
==========================================
- Coverage   89.09%   85.47%   -3.62%     
==========================================
  Files         109      109              
  Lines        3721     3721              
  Branches      348      348              
==========================================
- Hits         3315     3180     -135     
- Misses        365      476     +111     
- Partials       41       65      +24     

[mikehardy]

Hey @vonovak - haven't you contributed here before? I really thought you had - but the CLA is reporting it is not signed 🤔 ? https://cla-assistant.io/invertase/react-native-firebase?pullRequest=5000

[vonovak]

@mikehardy I just signed it. I remember I contributed some bit, long time ago. Maybe it was before there was the need to sign anything :)

Thanks!