doc: add prompt to fix repl_test.js example
Running repl.start without the prompt set produces this error:
repl.js:95
throw new Error('An options Object, or a prompt String are required');
^
Error: An options Object, or a prompt String are required
at new REPLServer (repl.js:95:11)
at Object.exports.start (repl.js:321:14)
at Object.<anonymous> (/Users/dan/Dropbox/Documents/dev/nextgen/repl_test.js:5:6)
at Module._compile (module.js:449:26)
at Object.Module._extensions..js (module.js:467:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Module.runMain (module.js:492:10)
at process.startup.processNextTick.process._tickCallback (node.js:244:9)