docs: ABI: fix an RST error in sysfs-bus-nvdimm

The literal blocks in this file lacked the necessary blank line at the top,
causing Sphinx to complain:

  /Documentation/ABI/testing/sysfs-bus-nvdimm:11: WARNING: Unexpected indentation.

Add the lines it's longing for and bring about a bit of warning peace.
Reported-by: Hu Haowen <src.res.211@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/sysfs-bus-nvdimm

@@ -18,10 +18,12 @@ Description:	(RO) Attribute group to describe the magic bits
 		Each attribute under this group defines a bit range of the
 		perf_event_attr.config. Supported attribute is listed
 		below::
+
 		  event  = "config:0-4"  - event ID
 
 		For example::
-			ctl_res_cnt = "event=0x1"
+
+		  ctl_res_cnt = "event=0x1"
 
 What:           /sys/bus/event_source/devices/nmemX/events
 Date:           February 2022