Bump 'src/doc/book' git submodule.

[frewsxcv]

Primarily to pick up this change:

rust-lang/book#866

...to move this PR forward:

#43641

[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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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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[carols10cents]

@bors r+

[bors]

📌 Commit fd253b5 has been approved by carols10cents

[frewsxcv]

@bors rollup

[bors]

⌛ Testing commit fd253b5e3bbb62892a9b2989f92594d285a2cbb5 with merge 25015a2bee498fa675f0632d35ed2fa147850962...

[kennytm]

Tidy failure. Please update the Book again.

[00:04:05] tidy error: /checkout/src/doc/book/second-edition/tools/src/bin/convert_quotes.rs:48: line longer than 100 chars
[00:04:05] tidy error: /checkout/src/doc/book/second-edition/tools/src/bin/convert_quotes.rs:56: line longer than 100 chars
[00:04:05] tidy error: /checkout/src/doc/book/second-edition/tools/src/bin/convert_quotes.rs: incorrect license
[00:04:05] some tidy checks failed

[bors]

💔 Test failed - status-travis

[frewsxcv]

Waiting on rust-lang/book#868

[carols10cents]

@bors: r+

[bors]

📌 Commit abc76ae has been approved by carols10cents

[frewsxcv]

Thanks for bumping. Haven't been at my computer for a while

[carols10cents]

Sorry I wasn't at my computer for a while to merge the 2nd fix :)

[bors]

⌛ Testing commit abc76ae with merge dd1df35...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: carols10cents
Pushing dd1df35 to master...