Update 2-promise.html: render correct message

__Problem__:
The current implementation of `2-promise.html` yields `[object Object] instead of `Hello World` string.

__Solution__:
Rewrite the callback to yield the expected message.

__This PR__:
* Renames the callback's first argument to "data" representing the JSON object data.
* The data itself is a plain JS object with two keys (message, status). `var message` accesses the data's message key, allowing the callback to yield the expected message.