Fixed an error in the ObjectValidator typings (#13789)

The correct method-name has always been `withCustom()` instead of `custom()`. Also, the method-signature is (sadly) inconsistent with the other `withX()`-methods, since it expects the `ValidateFn` directly instead of wrapped in a `Validatable` (but it does wraps it into a `Validatable` internally).

node-validator/index.d.ts

@@ -32,7 +32,7 @@ declare namespace Validator {
     interface IsObjectValidator extends Validatable {
         withRequired: (name: String, validator: Validatable) => IsObjectValidator,
         withOptional: (name: String, validator: Validatable) => IsObjectValidator,
-        custom: (customValidator: Validatable) => IsObjectValidator,
+        withCustom: (customValidator: ValidateFn) => IsObjectValidator,
         validate: ValidateFn
     }