fix ref

Author: mongoKart

source/whats-new.txt

@@ -40,7 +40,7 @@ What's New in 3.0
 
 .. warning:: Breaking Changes in v3.0
 
-   The v3.0 driver contains breaking changes. See :ref:`<csharp-breaking-changes-3.0>`
+   The v3.0 driver contains breaking changes. See :ref:`<csharp-breaking-changes-v3>`
    for more information.
 
 The 3.0 driver release includes the following new features: