Update various book modules

This includes an important fix for rustc contributors in
https://github.com/rust-lang/book/pull/571

I'm going to update the other ones as well here while we're at it; no
need to spam PRs.

3 files changed, 0 insertions, 0 deletions

diff --git a/src/doc/book b/src/doc/book
-Subproject 9bd223ca406b1170a24942d6474f9e8a56f4a42
+Subproject a2c56870d4dc589237102cc5e0fe7b9ebd0d14a
diff --git a/src/doc/nomicon b/src/doc/nomicon
-Subproject d08fe97d12b41c1ed8cc7701e54586413278394
+Subproject 616b98444ff4eb5260deee95ee3e090dfd98b94
diff --git a/src/doc/reference b/src/doc/reference
-Subproject 516549972d61c8946542d1a34afeae97167ff77
+Subproject acedc32cacae80cf2f4925753a4ce7f7ffd7c86