[Documentation] iframe url for local tophatting in Playground is incorrect

[brendanatshopify]

Issue summary

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.

Expected behavior

While running Storybook locally this url should bring up an isolated view of the Playground component.

Actual behavior

While running Storybook locally this brings up an error dialogue:

Steps to reproduce the problem

1. Run yarn dev to start Storybook
2. Visit http://YOUR_IP_ADDRESS:ASSIGNED_PORT/iframe.html?selectedKind=Playground&selectedStory=Playground
3. You should see the error in the above screenshot

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