fix stable diffusion app

[kning]

Currently the deployed app fails because it fails to find FastAPI see #569

Type of Change

• New Example
• Example updates (Bug fixes, new features, etc.)
• Other (changes to the codebase, but not to examples)

Checklist

• Example is testable in synthetic monitoring system, or lambda-test: false is added to example frontmatter
• Example does not require third-party dependencies to be installed locally
• Example pins all dependencies and specifies a python_version for the base image
• Example is documented with comments throughout, in a Literate Programming style.

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