fix broken link (#5122)

Signed-off-by: nikki everett <[email protected]>

docs/user_guide/data_types_and_io/index.md

@@ -133,7 +133,7 @@ Here's a breakdown of these mappings:
       - Any
       - Custom transformers
       - The ``FlytePickle`` transformer is the default option, but you can also define custom transformers.
-        **For instructions on building custom type transformers, please refer to :ref:`this section <advanced_custom_types>`**.
+        For instructions on building custom type transformers, please refer to :ref:`this section <advanced_custom_types>`.
 ```
 
 ```{toctree}