sensor.alpha_vantage: removed Google as default stock

[ChristianKuehnel]

Description:
after the change to the code, we also need to remove the default stock Google from the documentation

Pull request in home-assistant (if applicable): home-assistant/core#12252

Checklist:

• Branch: Fixes, changes and adjustments should be created against current. New documentation for platforms/components and features should go to next.
• The documentation follow the standards.

[arsaboo]

The config indicated

sensor:
  - platform: alpha_vantage
    api_key: YOUR_API_KEY

is no longer complete. We need to indicate that one of them (forex or stocks) is required.

[arsaboo]

Looks good...can me merged when the parent PR is merged 👍

[fabaff]

Thanks 🐦