update the disk.json version for the 2021-03-01 (Azure#13461)

Co-authored-by: Meet Shah <[email protected]>

Author: 2 people

specification/compute/resource-manager/readme.md

@@ -223,7 +223,7 @@ input-file:
 - Microsoft.Compute/stable/2020-12-01/compute.json
 - Microsoft.Compute/stable/2020-12-01/runCommands.json
 - Microsoft.Compute/stable/2019-04-01/skus.json
-- Microsoft.Compute/stable/2020-09-30/disk.json
+- Microsoft.Compute/stable/2020-12-01/disk.json
 - Microsoft.Compute/stable/2019-12-01/gallery.json
 - Microsoft.Compute/stable/2021-03-01/cloudService.json
 ```