Update documentation for the Bound Mutate example

[hytea]

This is a link to the documentation that I found incorrect: https://swr.vercel.app/docs/mutation#bound-mutate

A bound mutate method requires a KeyedMutator which is either Data or a Promise to return Data.

This issue actually gives a good example of the todos. You may prefer to update this whole page to use the todo example.

Description

I was trying to follow the example in the Bound mutator section of the documentation and came across this example which is wrong. A bound mutator requires a KeyedMutator which requires Data or a Promise to return Data.

• Adding new page
• Updating existing documentation
• Other updates

[vercel]

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