x86: Documentation/i386/IO-APIC.txt: fix description

The description of the interrupt routing doesn't match the (nice) diagram.

Signed-off-by: Nick Andrew <nick@nick-andrew.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>