Fix a typo in guide

[vrana]

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 @pcwalton (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. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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 CONTRIBUTING.md for more information.

[huonw]

Thanks a lot for the contributions!

By the way, could I request that you maybe batch up the changes a little more tightly in future? (Having a lot of one line PRs makes it easy for things to get lost, and generally places a little bit more load on our reviewers and infrastructure.)

r? @steveklabnik

[sinistersnare]

@huonw hes using the web interface, which makes it hard to batch, as I understand it.