Fixed incorrect js selector for `\yii\widgets\ActiveForm::errorSummaryCssClass`

Example:

```php
$form = ActiveForm::begin([
    'errorSummaryCssClass' => 'some-class',
]);
```

Works fine! While:

```php
$form = ActiveForm::begin([
    'errorSummaryCssClass' => 'some-class-1 some-class-2',
]);
```

Does not works. PR fix that.

It works as it is. There is no need care about js selector so much... jQuery handle situations like: `.foo .bar`, ` .foo.bar ` or ` .foo .bar ` without any problem. So using `preg_split` here will be overcoding.