Fix minor syntax error in Rustonomicon Rc example.

[solson]

r? @steveklabnik

[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.

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[alexcrichton]

@bors: r+ 8222970

Thanks!

[steveklabnik]

@bors: rollup