[Doc] Update exporter usage

[ohbarye]

For https://marmelab.com/react-admin/doc/3.10/List.html#exporter

According to the type definition, Exporter is a function that returns Promise<void>.

# https://github.com/marmelab/react-admin/blob/9cc5e83252455b5e3e172306419431716f17a057/packages/ra-core/src/types.ts#L492-L501
export type Exporter = (
    data: any,
    fetchRelatedRecords: (
        data: any,
        field: string,
        resource: string
    ) => Promise<any>,
    dataProvider: DataProvider,
    resource?: string
) => Promise<void>;

And jsonexport that used in the example is a function that returns void.
So the example's return value does not match Exporter type without wrapping with Promise.

# https://github.com/DefinitelyTyped/DefinitelyTyped/blob/40e88087e9ec5898250644da6e4653888905bd85/types/jsonexport/index.d.ts#L112-L116
declare function jsonexport(
    json: object | object[],
    userOptions: jsonexport.UserOptionsWithHandlers,
    cb: (err: Error, csv: string) => void,
): void;

[fzaninotto]

The type definition is wrong. the return type should be void|Promise<void>. Would you like to change it instead?

[ohbarye]

Got it, thanks. Could you check #5782 instead?