Use tabs to indent the graph documentation, not spaces.

Fixes: f2dde1ed ("Documentation: ACPI: move dsd/graph.txt to firmware-guide/acpi and convert to reST")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>