docs: powerpc: cxl.rst: mark two section titles as such

The User API chapter contains two sub-chapters. Mark them as
such.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Link: https://lore.kernel.org/r/190d67397cd63e419de8d85b92e8018d48e8c345.1586881715.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/powerpc/cxl.rst

@@ -133,6 +133,7 @@ User API
 ========
 
 1. AFU character devices
+^^^^^^^^^^^^^^^^^^^^^^^^
 
     For AFUs operating in AFU directed mode, two character device
     files will be created. /dev/cxl/afu0.0m will correspond to a
@@ -395,6 +396,7 @@ read
 
 
 2. Card character device (powerVM guest only)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
     In a powerVM guest, an extra character device is created for the
     card. The device is only used to write (flash) a new image on the