Updates element argument type in react-ssr-prepass.d.ts

[TheSonOfThomp]

Resolves #86

The type defined in Flow did not match the Typescript equivalent in @types/react. I've updated the .d.ts file to reference the correct TS type

// src/element.js
export type UserElement = {
  type: ComponentType<DefaultProps> & ComponentStatics,
  props: DefaultProps,
  $$typeof: typeof REACT_ELEMENT_TYPE
}

// @types/react 
interface ReactElement<P = any, T extends string | JSXElementConstructor<any> = string | JSXElementConstructor<any>> {
  type: T;
  props: P;
  key: Key | null;
}