Fix a typo in head.go (#553)

Signed-off-by: zhulongcheng <zhulongcheng.me@gmail.com>

head.go

@@ -892,7 +892,7 @@ func (h *Head) Delete(mint, maxt int64, ms ...labels.Matcher) error {
 	if h.wal != nil {
 		// Although we don't store the stones in the head
 		// we need to write them to the WAL to mark these as deleted
-		// after a restart while loeading the WAL.
+		// after a restart while loading the WAL.
 		if err := h.wal.Log(enc.Tombstones(stones, nil)); err != nil {
 			return err
 		}