[LAD] Update version in README.md
# Diagnostic Extension
Allow the owner of the Azure Virtual Machines to obtain diagnostic data for a Linux virtual machine.
-Latest version is 2.3.9019.
+Latest version is 2.3.9021.
You can read the User Guide below for detail:
* [Use the Linux Diagnostic Extension to monitor the performance and diagnostic data of a Linux VM](https://azure.microsoft.com/en-us/documentation/articles/virtual-machines-linux-diagnostic-extension/)
