doc: fix examples in repl.md

* Update an example according to an actual REPL session.
* Replace an arrow function with a common function to hold `this`.

PR-URL: https://github.com/nodejs/node/pull/12684
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/repl.md

@@ -341,7 +341,7 @@ replServer.defineCommand('sayhello', {
     this.displayPrompt();
   }
 });
-replServer.defineCommand('saybye', () => {
+replServer.defineCommand('saybye', function saybye() {
   console.log('Goodbye!');
   this.close();
 });
@@ -448,6 +448,8 @@ without passing any arguments (or by passing the `-i` argument):
 ```js
 $ node
 > const a = [1, 2, 3];
+undefined
+> a
 [ 1, 2, 3 ]
 > a.forEach((v) => {
 ...   console.log(v);