Update api.md (#9429)
Update api.md (#9429)

Fix timestamp in example in exemplars query. The year was `020` instead of `2020`.

Signed-off-by: Ivana <80044ae9d30274cdea4c3101b199c72442e042dc@gmail.com>
keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,alileza/prometheus