x86/irq: Fix comment on IRQ vector layout

commit f5a3562e ("x86/irq: Reserve a per CPU IDT vector for posted
MSIs") changed the first system vector from LOCAL_TIMER_VECTOR to
POSTED_MSI_NOTIFICATION_VECTOR. Reflect this change in the vector layout
comment as well.

However, instead of pointing to the specific vector, use the
FIRST_SYSTEM_VECTOR indirection which essentially refers to the same.

This avoids unnecessary modifications to the same comment whenever
additional system vectors get added.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Link: https://lore.kernel.org/all/20240812214318.2715360-1-sohil.mehta@intel.com