flutter getting starting guide is missing amplify init instructions

**Describe the content issue**:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?

**URL page where content issue is**:
https://docs.amplify.aws/flutter/start/getting-started/setup/

Open the linked Flutter getting-started setup page and inspect the surrounding setup instructions to identify where Amplify initialization belongs. Add the missing amplify init guidance to that page, then verify the documentation builds and the setup flow is complete.

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