add link to model_mod_check documentation

guide/how-to-test-your-model-mod-routines.rst

@@ -5,6 +5,7 @@ The program ``model_mod_check.f90`` can be used to test the routines
 individually before running them with *filter*. Add a ``mkmf_model_mod_check``
 and ``path_names_model_mod_check`` to your ``DART/models/your_model/work``
 subdirectory. You might find it helpful to consult another model matching your
-model type (simple or complex). See the documentation for ``model_mod_check`` in
+model type (simple or complex). See the documentation for :doc:`model_mod_check 
+<../assimilation_code/programs/model_mod_check/model_mod_check>` in
 ``DART/assimilation_code/programs/model_mod_check`` for more information on the
 tests available.