docs: fix typo in xive doc

"Interrupt Pending Buffer" IPB, which got written as IBP due to typo.

The "IPB" register is also mentioned in same doc multiple times.

Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

docs/specs/ppc-xive.rst

@@ -157,7 +157,7 @@ Interrupt flow from an O/S perspective
 
 After an event data has been enqueued in the O/S Event Queue, the IVPE
 raises the bit corresponding to the priority of the pending interrupt
-in the register IBP (Interrupt Pending Buffer) to indicate that an
+in the register IPB (Interrupt Pending Buffer) to indicate that an
 event is pending in one of the 8 priority queues. The Pending
 Interrupt Priority Register (PIPR) is also updated using the IPB. This
 register represent the priority of the most favored pending