Addon-docs: Fix canvas support expand code for non-story

[youngboy]

Issue:

option withSource="open" describe in https://storybook.js.org/docs/react/writing-docs/doc-block-canvas not work for non-story content

What I did

How to test

<Canvas withSource="open">
  <h2>Some here</h2>
</Canvas>

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

[kylegach]

This looks great! Thanks, @youngboy!

And congratulations on your first contribution! 🥳

/cc @shilman