message: [Doc] update button document
diff: </demo-block>
  <demo-block :title="$t('plain')">
  - <van-button plain type="primary">{{ $t('warning') }}</van-button>
  - <van-button plain type="danger">{{ $t('danger') }}</van-button>
  + <van-button plain type="primary">{{ $t('plain') }}</van-button>
  + <van-button plain type="danger">{{ $t('plain') }}</van-button>
  </demo-block>
  <demo-block :title="$t('disabled')">
  