doc: Fix diagram references in memory-ordering document

The three diagrams describing rcu_gp_init() all spuriously refer to
the same figure, probably due to a copy/paste issue. This commit fixes
these references.

Signed-off-by: Frederic Weisbecker <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst

···
 leftmost ``rcu_node`` structure offlines its last CPU and if the next
 ``rcu_node`` structure has no online CPUs).
 
-.. kernel-figure:: TreeRCU-gp-init-1.svg
+.. kernel-figure:: TreeRCU-gp-init-2.svg
 
 The final ``rcu_gp_init()`` pass through the ``rcu_node`` tree traverses
 breadth-first, setting each ``rcu_node`` structure's ``->gp_seq`` field
 to the newly advanced value from the ``rcu_state`` structure, as shown
 in the following diagram.
 
-.. kernel-figure:: TreeRCU-gp-init-1.svg
+.. kernel-figure:: TreeRCU-gp-init-3.svg
 
 This change will also cause each CPU's next call to
 ``__note_gp_changes()`` to notice that a new grace period has started,