[url] Fix a minor typo (#91)

Replace `A` with `An`
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/url/pull/91

-- Commit Summary --

  * Fix a minor typo

-- File Changes --

    M url.bs (2)

-- Patch Links --

https://github.com/whatwg/url/pull/91.patch
https://github.com/whatwg/url/pull/91.diff

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https://github.com/whatwg/url/pull/91