Fix broken link to run_app_with_gemini_key.md in examples READMEs

[omarfarouk228]

Description

Fixes #783
The file was moved to docs/usage/ but the links in two example READMEs still pointed to the old path, causing a 404.

Fixed in:

• examples/travel_app/README.md
• examples/simple_chat/README.md

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[gemini-code-assist]

Code Review

This pull request fixes broken links to the run_app_with_gemini_key.md documentation in the simple_chat and travel_app example READMEs and updates the genui package changelog. I have no feedback to provide.

_{Note: Security Review has been skipped due to the limited scope of the PR.}