Update 2-promise.html: render correct message

Problem:
The current implementation of 2-promise.html yields `[object Object]` rather than the "Hello World" message.

Solution:
Rewrite the callback to yield the expected message.

* The callback's first argument is not the message but the data from the JSON object.
* The data itself is a plain JS object with two keys (message, status). The function-scoped "message" variable accesses the data's message key, allowing the callback to yield the expected message.
Merge pull request #40 from mizuhands/master

Update 2-promise.html: render correct message