Fix closing xml for sample node (#836)

Fixes dotnet/docs#11967

Author: Thraka

snippets/csharp/new-in-7/Program.cs

@@ -54,7 +54,7 @@ static void Main(string[] args)
             Console.WriteLine(item);
             item = 24;
             Console.WriteLine(matrix[4, 2]);
-            // <SnippetAssignRefReturn>
+            // </SnippetAssignRefReturn>
 
             Iterator.IteratorTest();
             Iterator.IteratorTestLocal();