Updating typing to include one alias

As per https://www.npmjs.com/package/jest-fetch-mock#mocking-multiple-fetches-with-different-responses

Author: mctaggaj

src/index.d.ts

@@ -12,6 +12,7 @@ declare module "jest-fetch-mock" {
     (input?: string | Request, init?: RequestInit): Promise<Response>;
     mockResponse(body: string, init?: MockParams): Fetch;
     mockResponseOnce(body: string, init?: MockParams): Fetch;
+    once(body: string, init?: MockParams): Fetch;
     mockResponses(...responses : Array<[string] | [string, MockParams]>): Fetch;
     mockReject(error?: Error): Fetch;
     mockRejectOnce(error?: Error): Fetch;