docs: Change type of 'tags' in annotationQuery result example to list
docs: Change type of 'tags' in annotationQuery result example to list

A datasource plugin must return a list of tags in the 'tags' field of
the annotationQuery result. A string 'test' would otherwise result in
4 tags in Grafana: 't','e','s','t'.

grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana