Update to currently output (gin-gonic#2188)

Excuse me, I forgot change output in gin-gonic#2186

README.md

@@ -754,7 +754,7 @@ $ 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=2018-03-10&check_out=2018-03-09"
-{"error":"Key: 'Booking.CheckIn' Error:Field validation for 'CheckIn' failed on the 'bookabledate' tag"}
+{"error":"Key: 'Booking.CheckOut' Error:Field validation for 'CheckOut' failed on the 'gtfield' tag"}
 ```
 
 [Struct level validations](https://github.com/go-playground/validator/releases/tag/v8.7) can also be registered this way.