[fetch] Removed promise antipattern from code example (#227)

See [What is the explicit promise construction antipattern and how do I avoid it?](https://stackoverflow.com/questions/23803743/what-is-the-explicit-promise-construction-antipattern-and-how-do-i-avoid-it) on StackOverflow.
Promise evangelists say it should be burned with fire, and not appear in any possibly educational code examples :-)
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/227

-- Commit Summary --

  * Removed promise antipattern from code example

-- File Changes --

    M Overview.src.html (25)

-- Patch Links --

https://github.com/whatwg/fetch/pull/227.patch
https://github.com/whatwg/fetch/pull/227.diff

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https://github.com/whatwg/fetch/pull/227