Fix a typo in head.go (#553)
Fix a typo in head.go (#553)

Signed-off-by: zhulongcheng <d0a909104c54f7c1064d09716f840641c5a2a6f9@gmail.com>
kangwoo/prometheus,Gouthamve/prometheus,prometheus/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,prometheus/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,prometheus/tsdb,alileza/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,prometheus/prometheus,kangwoo/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus