Change TypeScript import to match module definition

[rymndhng]

I noticed that the example in vanilla JS does not translate properly to TypeScript because the first argument client is actually expected to be the fetch module, rather than the fetch default export. This changes the type definitions to match how the module should be consumed.

[rymndhng]

I tested this by writing this typescript snippet that compiled correctly:

import createFetch from "@zeit/fetch";
import * as nodeFetch from "node-fetch";

export const fetch = createFetch(nodeFetch);
export default fetch;

[rymndhng]

Bumping on @javivelasco @TooTallNate. Is there anything preventing this PR from being merged?

[TooTallNate]

Thanks!