Merge pull request #480 from crd/policy_correction

Replace hardcoded incorrect reference to Chapter 6

Author: ben

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

@@ -427,7 +427,7 @@ target_shas.each do |sha|
   end
 end
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-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 <<_revision_selection>>. You get a list of commits that have already been pushed up by running this:
 
 [source,ruby]
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