chore: mutationFn should return a Promise type

[anc95]

As seen at https://react-query.tanstack.com/docs/api#usemutation,

mutationFn: Function(variables) => Promise
Required
A function that performs an asynchronous task and returns a promise.
variables is an object that mutate will pass to your mutationFn

I correct the demo code

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🎉 This PR is included in version 2.9.0 🎉

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