Update instructions to add model creation header

The first step is to create a model but the first header is "Create a Book Repository" which is the second step

README.adoc

@@ -64,7 +64,7 @@ include::complete/build.gradle[]
 ====
 
 [[initial]]
-== Create a Book Repository
+== Create a Book Model
 
 First, you need to create a simple model for your book. The following listing (from
 `src/main/java/com/example/caching/Book.java`) shows how to do so:
@@ -76,6 +76,8 @@ include::initial/src/main/java/com/example/caching/Book.java[]
 ----
 ====
 
+== Create a Book Repository
+
 You also need a repository for that model. The following listing (from
 `src/main/java/com/example/caching/BookRepository.java`) shows such a repository: