fix typedef for fetch to match spec (#717)

Co-authored-by: Henry Majoros <[email protected]>

Author: hmajoros

index.d.ts

@@ -6,7 +6,7 @@
 
 import RSVP from 'rsvp';
 export default function fetch(
-  input: RequestInfo,
+  input: URL | RequestInfo,
   init?: RequestInit
 ): RSVP.Promise<Response>;
 export const Headers: {