Update AutoRest C# version (#15145)

Author: azure-sdk

eng/Packages.Data.props

@@ -9,7 +9,7 @@
   <ItemGroup>
     <PackageReference Update="ApprovalTests" Version="3.0.22" />
     <PackageReference Update="ApprovalUtilities" Version="3.0.22" />
-    <PackageReference Update="AutoRest.CSharp.V3" Version="1.0.0-alpha.20200914.3" />
+    <PackageReference Update="AutoRest.CSharp.V3" Version="1.0.0-alpha.20200915.1" />
     <PackageReference Update="Azure.AI.FormRecognizer" Version="3.0.0" />
 		<PackageReference Update="Azure.AI.TextAnalytics" Version="5.0.0" />
     <PackageReference Update="Azure.Data.AppConfiguration" Version="1.0.0" />