net: sysfs: Fix /sys/class/net/<iface> path for statistics

The Documentation/ABI/testing/sysfs-class-net-statistics documentation
is pointing to the wrong path for the interface.  Documentation is
pointing to /sys/class/<iface>, instead of /sys/class/net/<iface>.

Fix it by adding the `net/` directory before the interface.

Fixes: 6044f970 ("net: sysfs: document /sys/class/net/statistics/*")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>