docs: fix typos and errors in core package README

[Zentex1337]

Summary

• Fix import statement to use @web3uikit/core instead of web3uikit to match installation instructions
• Add missing backtick before <Illustration /> component reference
• Fix unclosed </NotificationProvider> tag in code example
• Fix 'arbitary' to 'arbitrary' typo

Test plan

• Documentation changes only, no code changes
• Verified code examples are now correct
• Verified text reads correctly after changes

[changeset-bot]

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Latest commit: 97583c9

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