[whatwg/streams] fixup Markdown docs (#575)

It still mentioned TeeStream.md which got removed in #563.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/575

-- Commit Summary --

  * delete TeeStream.md mention in readme
  * fix typo in Contributing.md
  * use https URIs in docs

-- File Changes --

    M Contributing.md (2)
    M README.md (2)
    M Requirements.md (8)

-- Patch Links --

https://github.com/whatwg/streams/pull/575.patch
https://github.com/whatwg/streams/pull/575.diff

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