sysfs.txt was converted and renamed to sysfs.rst.

Update device_attr_show.cocci script accordingly.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/048ed24b09aefa0051d76396d6250e35e6ba035c.1599660067.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>