doc: replace anonymous functions in repl.md

Replaced with an object shorthand and an arrow function.

PR-URL: nodejs#10244
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/repl.md

@@ -328,14 +328,14 @@ const repl = require('repl');
 const replServer = repl.start({prompt: '> '});
 replServer.defineCommand('sayhello', {
   help: 'Say hello',
-  action: function(name) {
+  action(name) {
     this.lineParser.reset();
     this.bufferedCommand = '';
     console.log(`Hello, ${name}!`);
     this.displayPrompt();
   }
 });
-replServer.defineCommand('saybye', function() {
+replServer.defineCommand('saybye', () => {
   console.log('Goodbye!');
   this.close();
 });