tsdb: fix unkown ref in log (#8048)

Signed-off-by: JinLingChristopher <jinl1037@hotmail.com>

tsdb/head.go

@@ -626,7 +626,7 @@ Outer:
 	}
 
 	if unknownRefs.Load() > 0 {
-		level.Warn(h.logger).Log("msg", "Unknown series references", "count", unknownRefs)
+		level.Warn(h.logger).Log("msg", "Unknown series references", "count", unknownRefs.Load())
 	}
 	return nil
 }