docs: ABI: sysfs-class-net: fix a typo

clas->class

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4bba2a1592df5a9435c8d4757a9abf20246e2a99.1604312590.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>