KVM: arm64: Fix Function ID typo for PTP_KVM service

Per include/linux/arm-smccc.h, the Function ID of PTP_KVM service is
defined as ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID. Fix the typo in
documentation to keep the git grep consistent.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210417113804.1992-1-yuzenghui@huawei.com