docs: pmu: update a reference to a deprecated property name

event-to-mhpmevent was deprecated and replaced by
riscv,event-to-mhpmevent, but a reference remains to the old name.
Replace it with the new one.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>