fix: update text in git-enforced policy file

Author: crazy-poncho

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

@@ -123,7 +123,7 @@ In this case, you have a couple of administrators, some documentation writers wi
 
 [source]
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-avail|nickh,pjhyett,defunkt,tpw
+avail|nickh,pjhyett,defunkt,tpw|nil
 avail|usinclair,cdickens,ebronte|doc
 avail|schacon|lib
 avail|schacon|tests