Very minor spelling fix in the book

[ErikUggeldahl]

Changed datastructure to data structure.

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.

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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

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[ErikUggeldahl]

A brief search shows that this is the only page that uses this spelling, so we should be good after this.

[ErikUggeldahl]

Also, I should take the opportunity to thank you @steveklabnik for all of your hard work on documentation. I'm looking forward to the next iteration of the book, and I wish you and Carol all the success in writing it!

[apasel422]

@bors: r+ rollup

Thanks!

[bors]

📌 Commit 6976991 has been approved by apasel422

[steveklabnik]

Thanks 😄