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.

should use `implode('.', preg_split('/\s+/', $this->errorSummaryCssClass, -1, PREG_SPLIT_NO_EMPTY))`