Fix "Custom Validators" example (gin-gonic#2186)

* Update fixed error code from merged commit

According to [this](gin-gonic/examples@874dcfa) merged commit.

* Fixed incorrect testing date.

Original testing date incompatible demo require, can't get expect result.
check_in date need NOT AFTER check_out date.

README.md

@@ -704,25 +704,22 @@ package main
 
 import (
 	"net/http"
-	"reflect"
 	"time"
 
 	"github.com/gin-gonic/gin"
 	"github.com/gin-gonic/gin/binding"
-	"gopkg.in/go-playground/validator.v8"
+	"gopkg.in/go-playground/validator.v9"
 )
 
 // Booking contains binded and validated data.
 type Booking struct {
-	CheckIn  time.Time `form:"check_in" binding:"required,bookabledate" time_format:"2006-01-02"`
+	CheckIn  time.Time `form:"check_in" binding:"required" time_format:"2006-01-02"`
 	CheckOut time.Time `form:"check_out" binding:"required,gtfield=CheckIn" time_format:"2006-01-02"`
 }
 
-func bookableDate(
-	v *validator.Validate, topStruct reflect.Value, currentStructOrField reflect.Value,
-	field reflect.Value, fieldType reflect.Type, fieldKind reflect.Kind, param string,
-) bool {
-	if date, ok := field.Interface().(time.Time); ok {
+var bookableDate validator.Func = func(fl validator.FieldLevel) bool {
+	date, ok := fl.Field().Interface().(time.Time)
+	if ok {
 		today := time.Now()
 		if today.After(date) {
 			return false
@@ -756,7 +753,7 @@ func getBookable(c *gin.Context) {
 $ curl "localhost:8085/bookable?check_in=2018-04-16&check_out=2018-04-17"
 {"message":"Booking dates are valid!"}
 
-$ curl "localhost:8085/bookable?check_in=2018-03-08&check_out=2018-03-09"
+$ curl "localhost:8085/bookable?check_in=2018-03-10&check_out=2018-03-09"
 {"error":"Key: 'Booking.CheckIn' Error:Field validation for 'CheckIn' failed on the 'bookabledate' tag"}
 ```