Documentation: ACPI: EINJ: Fix obsolete example

Since commit 488dac0c ("libfs: fix error cast of negative value in
simple_attr_write()"), the EINJ debugfs interface no longer accepts
negative values as input. Attempt to do so will result in EINVAL.

Fixes: 488dac0c

 ("libfs: fix error cast of negative value in simple_attr_write()")
Signed-off-by: Qifu Zhang <zhangqifu@bytedance.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>