fix(firestore, web): fix a casting error with the update method

[Lyokone]

Description

With the added support of updating fields containing a "." using FieldPath in update, a mistake was done during the casting of FieldPath.

This PR does not add tests because the case was already covered. There is an issue with the Firestore CI not catching the error. The method is properly failing on master locally before those changes.

CI being fixed here: #10461

Related Issues

fixes #10451

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