Bump 'src/doc/book' git submodule.

frewsxcv · carols10cents · commit abc76ae7e98b · 2017-08-05T16:29:10.000-04:00
Primarily to pick up this change: rust-lang/book#866 ...to move this PR forward: #43641
diff --git a/src/doc/book b/src/doc/book
@@ -1 +1 @@
-Subproject commit 4ee596df22f8ecaa9a0b2ddc0624b0104540dbf7
+Subproject commit 6f1a03dae6bcea44976918186f2d554186b3499c
