M10669: Possible typo mistake: 'SC1721' (dotnet#3167)

@wiwagn,
It seems that 'SC1721' is a typo of 'CS1721'. Could you help to review this and reply, please?
Many thanks in advance!

Author: carolinacmoravia

docs/csharp/language-reference/compiler-messages/cs1721.md

@@ -36,7 +36,7 @@ Class 'class' cannot have multiple base classes: 'class_1' and 'class_2'
  The most common cause of this error message is attempting to use multiple inheritance. A class in C# may only inherit directly from one class. However, a class can implement any number of interfaces.  
 
 ## Example  
- The following example shows one way in which SC1721 is generated, and then shows two possible ways to avoid the error.  
+ The following example shows one way in which CS1721 is generated, and then shows two possible ways to avoid the error.  
 
 ```csharp  
 // CS1721.cs