[whatwg/fetch] Update WPT link (#745)

I opted for the link text to be `web-platform-tests/wpt` to be more descriptive than `wpt`.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/745

-- Commit Summary --

  * Update WPT link

-- File Changes --

    M README.md (2)

-- Patch Links --

https://github.com/whatwg/fetch/pull/745.patch
https://github.com/whatwg/fetch/pull/745.diff

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