docs: Correct link to perunnodeui README.md

[octalmage]

Small fix in the main readme, the link to the tutorial doesn't work unless the file name is all caps.

Category

Docs

Testing

Clicking on the link in my branch loads the tutorial.

Checklist

• [ ] Name is added to the NOTICE file, if it is not present already.
• [x] Changes are rebased onto the target branch.

I don't feel like my name needs to be added to the NOTICE file, if this is inappropriate feel free to close the PR and make the change yourself.

Thank you!