Fixes #3693 Add API Key to doc

Author: ddobrin

models/spring-ai-google-genai/README.md

@@ -21,4 +21,5 @@
 export GOOGLE_GENAI_USE_VERTEXAI=true  
 export GOOGLE_CLOUD_PROJECT='your-project-id'  
 export GOOGLE_CLOUD_LOCATION='your-region'
+export GOOGLE_API_KEY='your-api-key'
 ```