Fix example code

Hey, I didn't try the lib yet but reading the example it seems that you need to change ```state``` to ```prevState``` here:

```typescript
// Or you can update it like React.useState's update
function decrement() {
store.set(prevState => ({
...state,
count: state.count - 1
});
}
```

Find the example containing the TypeScript decrement snippet and compare its update callback with React.useState's update form. Correct the example so it is valid and consistent with the stated usage, then verify that the displayed code can be read and used without the reported state-name error.

Written by the indexing model from the issue text.