Fixed description for NodeOsDiskSize parameter (Azure#16716)

There was a wrong description for NodeOsDiskSize (the same as for the NodeCount) parameter.

Author: kamilkonderak

src/Aks/Aks/help/Set-AzAksCluster.md

@@ -497,7 +497,7 @@ Accept wildcard characters: False
 ```
 
 ### -NodeOsDiskSize
-The default number of nodes for the node pools.
+Specifies the size, in GB, of the operating system disk.
 
 ```yaml
 Type: System.Int32