Update README.md

Author: penglongli

README.md

@@ -86,7 +86,7 @@ And you should define a `Gauge` Metric first,
 
 ```go
 gaugeMetric := &ginmetrics.Metric{
-    Type:        ginmetrics.Counter,
+    Type:        ginmetrics.Gauge,
     Name:        "example_gauge_metric",
     Description: "an example of gauge type metric",
     Labels:      []string{"label1"},