Fixes validate html to allow using multiple h1 tags

I added relaxerror rule to allow using multiple h1 tags for fixing npm test:

```
Running "validation:files" (validation) task
Validation started for.. _site/about/index.html
>> Validation successful..
Validation started for.. _site/components/index.html
>> Validation successful..
Validation started for.. _site/examples/app-android-notes/index.html
>> Validation successful..
Validation started for.. _site/examples/app-ios-mail/inbox.html
>> Validation successful..
Validation started for.. _site/examples/app-ios-mail/index.html
>> Validation successful..
Validation started for.. _site/examples/app-movies/choose-theater.html
>> Validation successful..
Validation started for.. _site/examples/app-movies/index.html
>> Validation successful..
Validation started for.. _site/examples/index.html
>> Validation successful..
Validation started for.. _site/getting-started/index.html
>> Validation successful..
Validation started for.. _site/index.html
>> Validation successful..
Validation started for.. _site/one.html
>> Validation successful..
Validation started for.. _site/template.html
>> Validation successful..
Validation started for.. _site/two.html
>> Validation successful..
Validation report generated: validation-report.json

Done, without errors.
```