Fix example code of Bind Uri (#2710)

Need to pass a string to `gin.H` to show a message correctly.

README.md

@@ -930,7 +930,7 @@ func main() {
 	route.GET("/:name/:id", func(c *gin.Context) {
 		var person Person
 		if err := c.ShouldBindUri(&person); err != nil {
-			c.JSON(400, gin.H{"msg": err})
+			c.JSON(400, gin.H{"msg": err.Error()})
 			return
 		}
 		c.JSON(200, gin.H{"name": person.Name, "uuid": person.ID})