Update autorest local version. (#945)

Update Autorest local version reference.

Author: finsharp

src/readme.graph.md

@@ -6,7 +6,7 @@
 azure: false
 powershell: true
 version: latest
-use: "./assets/autorest-powershell-2.1.402.tgz"
+use: "./assets/autorest-powershell-2.1.500.tgz"
 metadata:
     authors: Microsoft Corporation
     owners: Microsoft Corporation