Update font-optimization.md

[peraltafederico]

This PR fixes the documentation of @next/font/local. In the current documentation, className is called from localFont which comes from @next/font/local instead of doing it from the result of calling localFont itself.

Bug

• [ ] Related issues linked using fixes #number
• [ ] Integration tests added
• [ ] Errors have a helpful link attached, see contributing.md

Feature

• [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• [ ] Related issues linked using fixes #number
• [ ] Integration tests added
• [ ] Documentation added
• [ ] Telemetry added. In case of a feature if it's used or not.
• [ ] Errors have a helpful link attached, see contributing.md

Documentation / Examples

• [x] Make sure the linting passes by running pnpm build && pnpm lint
• [x] The "examples guidelines" are followed from our contributing doc