fix link (#513)

Author: mongoKart

source/whats-new.txt

@@ -89,7 +89,8 @@ The 3.1 driver release includes the following new features:
   in the Atlas Search documentation.
 
 - Adds support for the ``sort`` option to be passed to update commands. To learn more about
-  using update commands with the {+driver-short+}, see :ref:`csharp-change-guide`.
+  using update commands with the {+driver-short+}, see :ref:`csharp-update-one` and
+  :ref:`csharp-update-many`.
 
 For more information about this release, see the :github:`v3.1 release notes
 </mongodb/mongo-csharp-driver/releases/tag/v3.1.0>`.