Bump 'src/doc/book' git submodule.
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

aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust