ACPI / Documentation: add copy_dsdt to ACPI format options

commit aa2110cb (ACPI: add boot option acpi=copy_dsdt to fix corrupt
DSDT) added copy_dsdt as an ACPI boot option, but did not add it to ACPI
format options in Documentation/kernel-parameters.txt.  Fix that.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>