[Documentation] Update incorrect url for Playground tophatting inside iframe

[brendanatshopify]

WHY are these changes introduced?

Fixes: #4391

In the current project README.md the instructions contain a link to tophat the Playground in an isolated environment at the url: http://YOUR_IP_ADDRESS:ASSIGNED_PORT/iframe.html?selectedKind=Playground&selectedStory=Playground. This link brings up an error dialogue.

WHAT is this pull request doing?

This updates the documentation to use the path param as suggested by Storybook.

To 🎩 :

1. Open up Playground.tsx and paste the following in:

import React from 'react';

import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      <span role="img" aria-label="Hand wave emoji">
        👋
      </span>
    </Page>
  );
}

2. Run yarn dev
3. Visit http://YOUR_IP_ADDRESS:ASSIGNED_PORT/iframe.html?selectedKind=Playground&selectedStory=Playground
4. This should return an error dialogue
5. Visit http://YOUR_IP_ADDRESS:ASSIGNED_PORT/iframe.html?path=/story/playground-playground--playground
6. You should see the contents of the Playground on the page

[github-actions]

size-limit report

Path	Size
cjs	142.51 KB (0%)
esm	96.27 KB (0%)
esnext	139.44 KB (0%)
css	33.74 KB (0%)

[AndrewMusgrave]

Nice 🎉 Would you mind adding an addition to UNRELEASED.md