Update node's ReadLine.setPrompt to match new API

Fixes #5224

node/node.d.ts

@@ -781,7 +781,7 @@ declare module "readline" {
     import * as stream from "stream";
 
     export interface ReadLine extends events.EventEmitter {
-        setPrompt(prompt: string, length: number): void;
+        setPrompt(prompt: string): void;
         prompt(preserveCursor?: boolean): void;
         question(query: string, callback: Function): void;
         pause(): void;