Update book submodule to pass linkchecker test

kornelski · kornelski · commit 1912f39e7c02 · 2018-04-25T21:56:06.000+01:00
diff --git a/src/doc/book b/src/doc/book
@@ -1 +1 @@
-Subproject commit b889e1e30c5e9953834aa9fa6c982bb28df46ac9
+Subproject commit 6237a75790cd2e0ca22961b55f64a83319e73464
