docs(demo/readme): clarify demo docs regarding to issue #507

[Shooteger]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Description

This PR updates the README file in the "demo" folder as in #507

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

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[pkg-pr-new]

npm i https://pkg.pr.new/@nuxtjs/supabase@523

commit: f500800

[larbish]

Thanks!