acpi/doc: add missing param value

Add missing parameter value to list of available values
for acpi=<value>.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -134,7 +134,7 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	acpi=		[HW,ACPI,X86-64,i386]
 			Advanced Configuration and Power Interface
-			Format: { force | off | ht | strict | noirq }
+			Format: { force | off | ht | strict | noirq | rsdt }
 			force -- enable ACPI if default was off
 			off -- disable ACPI if default was on
 			noirq -- do not use ACPI for IRQ routing