Replace hardcoded incorrect reference to Chapter 6

Updated policy.asc in Chapter 8 to use a link to the relevant section in
Chapter 7 rather than a hardcoded and incorrect reference to Chapter 6.

Author: crd

book/08-customizing-git/sections/policy.asc

@@ -427,7 +427,7 @@ target_shas.each do |sha|
   end
 end
 ----
-This script uses a syntax that wasn't covered in the Revision Selection section of Chapter 6. You get a list of commits that have already been pushed up by running this:
+This script uses a syntax that wasn't covered in the <<_revision_selection>> section of <<_git_tools>>. You get a list of commits that have already been pushed up by running this:
 
 [source,ruby]
 ----