Bump 'src/doc/book' git submodule.

Primarily to pick up this change:

https://github.com/rust-lang/book/pull/866

...to move this PR forward:

https://github.com/rust-lang/rust/pull/43641
This commit is contained in:
Corey Farwell 2017-08-05 11:04:45 -04:00 committed by Carol (Nichols || Goulding)
parent 5c7add7551
commit abc76ae7e9

@ -1 +1 @@
Subproject commit 4ee596df22f8ecaa9a0b2ddc0624b0104540dbf7
Subproject commit 6f1a03dae6bcea44976918186f2d554186b3499c