Fix callback call in Fabric Trainer example

[liambsmith]

Fix for error:

TypeError: ModelHooks.on_before_optimizer_step() takes 2 positional arguments but 3 were given

The on_before_optimizer_step has no arguments, so the ", 0" part of the call caused this error.

What does this PR do?

Fixes #<issue_number>

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📚 Documentation preview 📚: https://pytorch-lightning--19986.org.readthedocs.build/en/19986/

[awaelchli]

Thanks @liambsmith!