correct `GET /api/annotations` path
correct `GET /api/annotations` path

This corrects the [GET /api/annotations](https://grafana.com/docs/grafana/latest/http_api/annotations/) URL path.

The correct path is `/api/annotations` and not `/api/annotation`.

For further context, see:
https://github.com/grafana/terraform-provider-grafana/issues/11#issuecomment-797424562

Signed-off-by: Mike Ball <9194f6a0a887c8836610cf16a18830148e95f911@gmail.com>

raintank/grafana-api-golang-client