[docs] Fix expected directory tree in _Shadowing other themes_

[damycra]

Description

Amended expected directory tree, purely based on the preceding text.

[johno]

Thanks for the PR! We should probably change the tree so it reads like:

user-site
└── src
    └── gatsby-plugin-theme-ui
        └──index.js // highlight-line

This is the more "correct" shadow path for the Theme UI plugin since it doesn't need to be nested under gatsby-theme-blog. Do you mind updating it and the path specified in the paragraph above?

[johno]

Ignore my last comment, didn't realize gatsby-theme-blog was using that path so this looks good to me. For the next major version of the theme we can flatten the pathing out.

Thanks!

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