KVM: arm64: Fix documentation for KVM_CAP_ARM_WRITABLE_IMP_ID_REGS

The capability actually fails with EINVAL if vCPUs have already been
created.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250305230825.484091-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Documentation/virt/kvm/api.rst

@@ -8264,7 +8264,7 @@ take care to differentiate between these cases.
 :Architectures: arm64
 :Target: VM
 :Parameters: None
-:Returns: 0 on success, -EBUSY if vCPUs have been created before enabling this
+:Returns: 0 on success, -EINVAL if vCPUs have been created before enabling this
           capability.
 
 This capability changes the behavior of the registers that identify a PE