[fetch] improve example (#157)

`res.headers.get("content-type")` could be `null` or `Application/json; Charset=utf-8`
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/157

-- Commit Summary --

  * improve example

-- File Changes --

    M Overview.src.html (3)

-- Patch Links --

https://github.com/whatwg/fetch/pull/157.patch
https://github.com/whatwg/fetch/pull/157.diff

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https://github.com/whatwg/fetch/pull/157