[docs] fix: migration-v4 `Select` component `onChange` event type

[tzynwang]

• [x] I have followed (at least) the PR section of the contributing guide.

Found out the type definition of onChange from Select component is actually SelectChangeEvent<T> instead of React.SyntheticEvent when migrating. Update the related parts in the docs.

updated doc preview: https://deploy-preview-35509--material-ui.netlify.app/material-ui/migration/v5-component-changes/#update-event-type-typescript-3

[mui-bot]

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:book:	Netlify deploy preview: https://deploy-preview-35509--material-ui.netlify.app/

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