fix typo

Author: wadave

examples/a2a-adk-app/host_agent/adk_agent/README.md

@@ -7,7 +7,7 @@ This example shows how to create an A2A Server that uses an ADK-based Agent.
 
 1. Create the .env file with your API Key
    ```bash
-   OOGLE_GENAI_USE_VERTEXAI=TRUE
+   GOOGLE_GENAI_USE_VERTEXAI=TRUE
    GOOGLE_CLOUD_PROJECT="your project id"
    GOOGLE_CLOUD_LOCATION=us-central1
    AIR_AGENT_URL=http://localhost:10002