Fix the homeserver config example in presence router docs (#10288)

The presence router docs include some sample homeserver config. At some point we changed the name of the [config option](https://github.com/matrix-org/synapse/blob/859dc05b3692a3672c1a0db8deaaa9274b6aa6f5/docs/sample_config.yaml#L104-L113), but forgot to update the docs.

I've also added `presence.enabled: true` to the example, as that's the new way to enable presence (the `presence_enabled` option has been deprecated).