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>