selectively demote headers for ebook generation

Author: jnavila

progit.asc

@@ -8,8 +8,8 @@ Pro Git
 :front-cover-image: image:book/cover.png[width=1050,height=1600]
 
 include::book/preface.asc[]
+ifdef::ebook-format[:leveloffset: -1]
 
-:leveloffset: -1
 include::book/01-introduction/1-introduction.asc[]
 
 include::book/02-git-basics/1-git-basics.asc[]