book: Add missing punctuation

[salty-horse]

Missing period at the end of a sentence.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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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[sanxiyn]

@bors r+ rollup

[bors]

📌 Commit bb3360a has been approved by sanxiyn

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit bb3360a has been approved by steveklabnik

[steveklabnik]

ha! just beat me to it, lol

[bors]

⌛ Testing commit bb3360a with merge 0cdcd4f...

[bors]

💔 Test failed - auto-mac-64-opt

[alexcrichton]

@bors: retry

On Sat, Dec 12, 2015 at 12:06 AM, bors notifications@github.com wrote:

[image: 💔] Test failed - auto-mac-64-opt
http://buildbot.rust-lang.org/builders/auto-mac-64-opt/builds/7369

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Reply to this email directly or view it on GitHub
#30329 (comment).

[bors]

⌛ Testing commit bb3360a with merge 5de242f...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt