Update index.mdx – Close [contribution guidelines] markdown link

[mlettini]

It was missing the closing parenthesis. Just a little thing I saw 👍

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1add358

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[vercel]

@mlettini is attempting to deploy a commit to the Lemon Squeezy Team on Vercel.

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[betegon]

whooops, i missed this and opened #87 . closing it now ;)