Fixes MicrosoftDocs/azure-docs#8974

The C# code sample has invalid variable passed to PlacementPolicies.Add() method in Preferred Domain section.

ServicePlacementPreferPrimaryDomainPolicyDescription primaryDomain = new ServicePlacementPreferPrimaryDomainPolicyDescription();
primaryDomain.DomainName = "fd:/EastUS/";
serviceDescription.PlacementPolicies.Add(invalidDomain); // <- should be "primaryDomain" here

Author: mimckitt

articles/service-fabric/service-fabric-cluster-resource-manager-advanced-placement-rules-placement-policies.md

@@ -87,7 +87,7 @@ The Preferred Primary Domain specifies the fault domain to place the Primary in.
 ```csharp
 ServicePlacementPreferPrimaryDomainPolicyDescription primaryDomain = new ServicePlacementPreferPrimaryDomainPolicyDescription();
 primaryDomain.DomainName = "fd:/EastUS/";
-serviceDescription.PlacementPolicies.Add(invalidDomain);
+serviceDescription.PlacementPolicies.Add(primaryDomain);
 ```
 
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