docs(ngModelController): update setValidity

Needs to be `$error[validationErrorKey]!=isValid` and not
`$error[validationErrorKey]=isValid`.

See https://github.com/angular/angular.js/blob/master/src/ng/directive/input.js#L1627

Closes angular#7934

Author: ephigabay

src/ng/directive/input.js

@@ -1598,7 +1598,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    *        `ngModel.$validators` pipeline which is designed to handle validations with true/false values.
    *
    * @param {string} validationErrorKey Name of the validator. the `validationErrorKey` will assign
-   *        to `$error[validationErrorKey]=isValid` so that it is available for data-binding.
+   *        to `$error[validationErrorKey]=!isValid` so that it is available for data-binding.
    *        The `validationErrorKey` should be in camelCase and will get converted into dash-case
    *        for class name. Example: `myError` will result in `ng-valid-my-error` and `ng-invalid-my-error`
    *        class and can be bound to as  `{{someForm.someControl.$error.myError}}` .