Fix typo in chrony configuration and add stratum to PTP device reference

From Azure docs https://learn.microsoft.com/en-us/azure/virtual-machines/linux/time-sync#chrony

Stratum information isn't automatically conveyed from the Azure host to the Linux guest. The preceding configuration line specifies that the Azure host time source is to be treated as Stratum 2, which in turn causes the Linux guest to report itself as Stratum 3.