Documentation/filesystems/sysfs.txt: fix device_attribute declaration

Fix a wrong device_attribute declaration example.

Signed-off-by: Andre Richter <andre.o.richter@gmail.com>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>