doc: cgroup: improve formatting of io example

We need a literal section, like few paragraphs below.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20200228000653.1572553-4-kuba@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/cgroup-v2.rst

@@ -1466,7 +1466,7 @@ IO Interface Files
 	  dios		Number of discard IOs
 	  ======	=====================
 
-	An example read output follows:
+	An example read output follows::
 
 	  8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0
 	  8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021