update Readme.md. Add code to String method

README.md

@@ -24,7 +24,7 @@ import "github.com/gin-gonic/gin"
 func main() {
     r := gin.Default()
     r.GET("ping", func(c *gin.Context){
-        c.String("pong")
+        c.String(200, "pong")
     })
     
     // Listen and server on 0.0.0.0:8080
@@ -287,4 +287,4 @@ func main() {
     }
     s.ListenAndServe()
 }
-```
+```