doc/pull: clarify the illustrations

The second illustration that shows the history after "git pull"
spelled the remote-tracking branch with "remotes/" prefix, which
is not necessary.  Drop it.

To match the assumption that a remote-tracking branch is used to
keep track of the advancement of the master at the origin, update
the first illustration that shows the history before "git pull"
to show the distinction between the master currently at origin and
the stale origin/master remote-tracking branch.

Noticed-by: Felipe Contreras <felipe.contreras@gmail.com>
Helped-by: Max Horn <max@quendi.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pull.txt

@@ -42,6 +42,8 @@ Assume the following history exists and the current branch is
 	  A---B---C master on origin
 	 /
     D---E---F---G master
+	^
+	origin/master in your repository
 ------------
 
 Then "`git pull`" will fetch and replay the changes from the remote
@@ -51,7 +53,7 @@ result in a new commit along with the names of the two parent commits
 and a log message from the user describing the changes.
 
 ------------
-	  A---B---C remotes/origin/master
+	  A---B---C origin/master
 	 /         \
     D---E---F---G---H master
 ------------