docs: cgroup-v1: use code block for locking order schema

The locking order schema is a figure (like diagram), which should have
been formatted with literal code block for consistency with other
figures.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: bagasme

Documentation/admin-guide/cgroup-v1/memory.rst

@@ -287,7 +287,7 @@ When oom event notifier is registered, event will be delivered.
 2.6 Locking
 -----------
 
-Lock order is as follows:
+Lock order is as follows::
 
   Page lock (PG_locked bit of page->flags)
     mm->page_table_lock or split pte_lock