docs(cloud_firestore): fix example dir path in CONTRIBUTING.md

[kichiemon]

Description

I edited CONTRIBUTING.md because of the example dir path was wrong.

• packages/cloud_firestore/example is not exists.
  • changed to packages/cloud_firestore/cloud_firestore/example

Related Issues

no exists.

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[kichiemon]

Just now I signed the CLA.

[kichiemon]

@googlebot I signed it!

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[Salakar]

LGTM - thank you for fixing this