scripts: device_attr_show.cocci: update location of sysfs doc

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>

scripts/coccinelle/api/device_attr_show.cocci

 // SPDX-License-Identifier: GPL-2.0-only
 ///
-/// From Documentation/filesystems/sysfs.txt:
+/// From Documentation/filesystems/sysfs.rst:
 ///  show() must not use snprintf() when formatting the value to be
 ///  returned to user space. If you can guarantee that an overflow
 ///  will never happen you can use sprintf() otherwise you must use