fix labels

Author: mongoKart

source/whats-new.txt

@@ -38,15 +38,15 @@ Learn what's new in:
 What's New in 3.1
 -----------------
 
-.. _csharp-version-3.1:
-
 The 3.1 driver release includes the following new features:
 
 - Adds new default serializers for immutable collections in the
   `System.Collections.Immutable <https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable?view=net-8.0>`__
   namespace. The driver can now serialize ``ImmutableArray`` objects, and serialization
   for other immutable collections is more memory efficient.
 
+.. _csharp-version-3.0:
+
 What's New in 3.0
 -----------------