Update boxing-and-unboxing.md (#5946)

The section calling attention to the c# spec was duplicated.

Author: CalvinAllen

docs/csharp/programming-guide/types/boxing-and-unboxing.md

@@ -107,8 +107,5 @@ int j = (int) o;
 
 -   [Value Types](../../../csharp/language-reference/keywords/value-types.md)  
 
-## C# Language Specification  
- [!INCLUDE[CSharplangspec](~/includes/csharplangspec-md.md)]  
-  
 ## See Also  
  [C# Programming Guide](../../../csharp/programming-guide/index.md)