[whatwg/streams] Editorial: fix missing async in example (#1076)

There's an `await` in the method body, so there should be an `async` in its signature.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/1076

-- Commit Summary --

  * Editorial: fix missing async in example

-- File Changes --

    M index.bs (2)

-- Patch Links --

https://github.com/whatwg/streams/pull/1076.patch
https://github.com/whatwg/streams/pull/1076.diff

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https://github.com/whatwg/streams/pull/1076