Fix reference to line number

Author: StoneyJackson

reference.md

@@ -201,7 +201,7 @@ $ git push origin master                               (28)
 Now that your work has been accepted in upstream, you can safely delete the
 branches you were working on both in your local and remote repositories (29-31). If you ever abandon your effort before
 a pull-request is accepted, you can also delete your branch; but you'll need
-to use -D (capital D) in (29).
+to use -D (capital D) in (30).
 
 ```bash
 $ git checkout master                                  (29)