Use '.js.map' as file extension for sourceMapName

SourceMap spec suggest use of `sourceFileName.targetExtension.map` format for sourcemap file.

- https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.txk3vdsh99hf

So I renamed `sourceMapName` in example code.

FYI, CoffeeScript change map filename with the same format.

- [SourceMap: Consistency in map filename · Issue #3297 · jashkenas/coffeescript](https://github.com/jashkenas/coffeescript/issues/3297 "SourceMap: Consistency in map filename · Issue #3297 · jashkenas/coffeescript")

Thanks.