add cross link

Author: rustagir

source/data-formats/serialization.txt

@@ -36,13 +36,11 @@ We recommend installing the ``bson-kotlinx`` library to support
 custom codecs that have configurations to encode defaults, nulls, and
 define class discriminators.
 
-.. TODO fix the link in the following note
-
 .. note::
 
    To learn how to use the ``Codec`` interface instead of the
    ``kotlinx.serialization`` library to specify custom
-   serialization behavior, see the Codecs guide.
+   serialization behavior, see the :ref:`kotlin-sync-codecs` guide.
 
    You might choose to use {+language+} serialization if you
    are already familiar with the library or if you prefer