docs: fix environment variable name for Gemini API

[lacolaco]

I followed the guide on genkit-angular-starter-kit/README.md but GEMINI_API_KEY env var doesn't work. As the error says, the correct var name is now GOOGLE_GENAI_API_KEY.

Non-streaming request failed with error: Please pass in the API key or set the GOOGLE_GENAI_API_KEY or GOOGLE_API_KEY environment variable.
For more details see https://firebase.google.com/docs/genkit/plugins/google-genai
Error: Please pass in the API key or set the GOOGLE_GENAI_API_KEY or GOOGLE_API_KEY environment variable.
For more details see https://firebase.google.com/docs/genkit/plugins/google-genai
    at defineGoogleAIModel2 (/Users/lacolaco/works/angular/examples/genkit-angular-starter-kit/.angular/cache/19.2.5/app/vite/deps_ssr/@genkit-ai_googleai.js:2699:15)

I confirmed it works with GOOGLE_GENAI_API_KEY on my machine.