Update `define.alias` API description
Update `define.alias` API description

The old readme says that the first parameter is the `new name` of `old path`, but according to the [test case](https://github.com/ember-cli/loader.js/blob/d5fb086ceb01296fd17bf32e7241f55501ded8d8/tests/all.js#L1837), the first parameter should be the `old path` and the second one is the `new name`.
ember-cli/loader.js,ember-cli/loader.js