Update durable-functions-dotnet-entities.md

the return type of Add function is Void, as we aren't returning anything.
Also in the Reset function, we are return Task.CompletedTask, but it's  return type is not defined.

Author: Ahmadzia307

articles/azure-functions/durable/durable-functions-dotnet-entities.md

@@ -122,12 +122,12 @@ public class Counter : TaskEntity<int>
         this.logger = logger; 
     }
 
-    public int Add(int amount) 
+    public void Add(int amount) 
     {
         this.State += amount;
     }
 
-    public Reset() 
+    public Task Reset() 
     {
         this.State = 0;
         return Task.CompletedTask;