Fix form validation docs example

[ConorCorp]

Description

The form example provided in the docs had 2 issues.

1. The custom validation message would not show on first submit because reportValidity wasn't called. Forcing the user to press a submit button 2x before seeing the message.
2. You couldn't submit the form after fixing the input because setCustomValidity was locked to the first error message. It needed to be cleared on keyup.

Corresponding issue:

Testing

Tested on chrome and brave with a form / submit button on the form.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• [] I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Looks good, thank you!