[Modal] update modal title styles

[laranoell]

Resolves issue:
Modal Title - Vertical alignment #868

Actual behavior

Proposed Change:

Bump margin-top up to 0.4rem (from 0.2rem)

WHAT is this pull request doing?

Slight change to margin top, modal header component

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import * as React from 'react';
import {Page} from '@shopify/polaris';

interface State {}

export default class Playground extends React.Component<never, State> {
  render() {
    return (
      <Page title="Playground">
        {/* Add the code you want to test here */}
      </Page>
    );
  }
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility

[ghost]

👋 Thanks for opening your first pull request. A contributor should give feedback soon. If you haven’t already, please check out the contributing guidelines. You can also join #polaris on the Shopify Partners Slack.

[dpersing]

Hi @laranoell! Thanks for this, it looks great. Could you add an entry to UNRELEASED.md?

[laranoell]

^ @dpersing added! Let me know if I should update the format (first time contributing to Polaris)

[dpersing]

Looks great! Go ahead and merge! And welcome. 😄

[ghost]

🎉 Thanks for your contribution to Polaris React!