docs(firebase_messaging): fix usage link to the documentation in the README.md

[adnanjpg]

Description

the Usage link in the README of firebase_messaging was pointing to the firestore documentation. this PR changes that link to point to the firebase_messaging documentation.

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[nilsreichardt]

@adnanjpg Thanks for your PR! ❤️ Can you add the type of your PR to follow the conventional commits? Something like "docs(firebaes_messaging): fix usage link to the documentation in the README.md"

[adnanjpg]

@nilsreichardt Sure, I've updated the title and the comment accordingly!