fix branch name to distinguish from directory

Author: tschady

book/07-git-tools/sections/subtree-merges.asc

@@ -48,7 +48,7 @@ This is sort of a strange concept. Not all the branches in your repository actua
 In this case, we want to pull the Rack project into our `master` project as a subdirectory.
 We can do that in Git with `git read-tree`.
 You’ll learn more about `read-tree` and its friends in <<_git_internals>>, but for now know that it reads the root tree of one branch into your current staging area and working directory.
-We just switched back to your `master` branch, and we pull the `rack` branch into the `rack` subdirectory of our `master` branch of our main project:
+We just switched back to your `master` branch, and we pull the `rack_branch` branch into the `rack` subdirectory of our `master` branch of our main project:
 
 [source,console]
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