fix: onConnect types (#3116)

types/dispatcher.d.ts

@@ -217,7 +217,7 @@ declare namespace Dispatcher {
   export type StreamFactory = (data: StreamFactoryData) => Writable;
   export interface DispatchHandlers {
     /** Invoked before request is dispatched on socket. May be invoked multiple times when a request is retried when the request at the head of the pipeline fails. */
-    onConnect?(abort: () => void): void;
+    onConnect?(abort: (err?: Error) => void): void;
     /** Invoked when an error has occurred. */
     onError?(err: Error): void;
     /** Invoked when request is upgraded either due to a `Upgrade` header or `CONNECT` method. */