Fix typo in index.mdx

[crystalbit]

Closes # (no issue)

📝 Description

Hi! I think there is a typo in documentation. Should be both theme or _theme

⛳️ Current behavior (updates)

Error when I paste the code: Cannot find name '_theme'. Did you mean 'theme'?

🚀 New behavior

No error

💣 Is this a breaking change (Yes/No):

No

📝 Additional Information

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Name	Status	Preview	Updated (UTC)
chakra-ui-docs	❌ Failed (Inspect)		Jan 9, 2024 6:23pm

[cschroeter]

@crystalbit

Thanks mate.