Documentation: ACPI: Prune DSDT override documentation from index

Commit d206cef0 ("ACPI: docs: Drop useless DSDT override documentation")
removes useless DSDT override documentation. However, the commit forgets
to prune the documentation entry from table of contents of ACPI admin
guide documentation, hence triggers Sphinx warning:

Documentation/admin-guide/acpi/index.rst:8: WARNING: toctree contains reference to nonexisting document 'admin-guide/acpi/dsdt-override'

Prune the entry to fix the warning.

Fixes: d206cef0 ("ACPI: docs: Drop useless DSDT override documentation")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>