x86: docs fixes to Documentation/i386/IO-APIC.txt

Clean up spelling and grammar of IO-APIC.txt

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