book: fix a typo

[kylog]

No description provided.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[steveklabnik]

@bors: r+ rollup

thank you!

[bors]

📌 Commit 38009bf has been approved by steveklabnik

[bors]

⌛ Testing commit 38009bf with merge 4d34170...

[bors]

💔 Test failed - auto-mac-64-opt-rustbuild

[alexcrichton]

@bors: retry

On Sat, Sep 10, 2016 at 6:03 PM, bors notifications@github.com wrote:

💔 Test failed - auto-mac-64-opt-rustbuild
https://buildbot.rust-lang.org/builders/auto-mac-64-opt-rustbuild/builds/2433

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