standardize variable in buildHeader()

In EntityListController we have:

  public function buildHeader() {
    $row['label'] = t('Label');
    $row['operations'] = t('Operations');
    return $row;
  }

but in VocabularyListController we have:

  /**
   * {@inheritdoc}
   */
  public function buildHeader() {
    $header['label'] = t('Vocabulary name');
    $header['weight'] = t('Weight');
    return $header + parent::buildHeader();
  }

Using a different variable name makes it harder to follow the method inheritance, and also means that it's harder to move code up and down between inherited methods.

I would suggest $header is better than $row as it's more expressive.