Fix issue progit#448: 3.2 Git Branching -- possibly wrong explanation…

… (upstream vs. downstream)

book/03-git-branching/sections/basic-branching-and-merging.asc

@@ -105,7 +105,7 @@ Fast-forward
 ----
 
 You'll notice the phrase ``fast-forward'' in that merge.
-Because the commit pointed to by the branch you merged in was directly upstream of the commit you're on, Git simply moves the pointer forward.
+Because the commit `C4` pointed to by the branch `hotfix` you merged in was directly ahead of the commit `C2` you're on, Git simply moves the pointer forward.
 To phrase that another way, when you try to merge one commit with a commit that can be reached by following the first commit's history, Git simplifies things by moving the pointer forward because there is no divergent work to merge together – this is called a ``fast-forward.''
 
 Your change is now in the snapshot of the commit pointed to by the `master` branch, and you can deploy the fix.