Fix Flow error introduced in 63ea52b

Now detected by Flow 0.47.0.

src/index.js

@@ -39,7 +39,7 @@ import type { $Request, $Response } from 'express';
  * that returns an Object or a Promise for an Object.
  */
 export type Options =
-  ((request: $Request, response: $Response) => OptionsResult)
+  ((request: $Request, response: $Response, params?: GraphQLParams) => OptionsResult)
   | OptionsResult;
 export type OptionsResult = OptionsData | Promise<OptionsData>;
 export type OptionsData = {