[xhr] Fixing small typo (#29)

![xhr-pull](https://cloud.githubusercontent.com/assets/712910/10564042/62f7fa46-75c1-11e5-8ab2-022f84189ece.png)

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/xhr/pull/29

-- Commit Summary --

  * Fixing small typo

-- File Changes --

    M Overview.src.html (2)

-- Patch Links --

https://github.com/whatwg/xhr/pull/29.patch
https://github.com/whatwg/xhr/pull/29.diff

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/pull/29