Correct placement of master for squash merge example

jmbtutor · jmbtutor · commit ea3317c649c5 · 2018-06-22T13:48:25.000-04:00
`master` should be on D, not B.
diff --git a/understanding-git.tex b/understanding-git.tex
@@ -1167,7 +1167,7 @@ \subsection{\gitcmd{merge}}
         }
       };
       \gitbranch{master}
-      {above=of B} {B}
+      {above=of D} {D}
       \gitbranch{topic}
       {right=of E} {E}
       \gitHEAD
