README contains incorrect template name for CLI command

  Description:
  The README.md incorrectly references `voice-assistant-frontend` as the template name, but the correct template name is `agent-starter-react`.

  **Current (incorrect):**
  ```bash
  lk app create --template voice-assistant-frontend

  Should be:
  lk app create --template agent-starter-react

  The template voice-assistant-frontend does not exist in the livekit-examples repository.