fix typo reported by https://github.com/kataras/iris/issues/1715

Model-validation.md

@@ -55,7 +55,7 @@ func main() {
 
         // Returns InvalidValidationError for bad validation input,
         // nil or ValidationErrors ( []FieldError )
-        err := vctx.ReadJSON(&user)
+        err := ctx.ReadJSON(&user)
         if err != nil {
             // This check is only needed when your code could produce
             // an invalid value for validation such as interface with nil