docs/sysfs: show() methods should use scnprintf().

Since snprintf() may return a value that exceeds its second argument,
show() methods should use scnprintf() instead of snprintf(). This patch
updates the example in the sysfs documentation accordingly.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>