[docs] fix broken SynapseML link (#4712)

* [docs] fix broken SynapseML link

* just remove link

docs/Parallel-Learning-Guide.rst

@@ -49,7 +49,7 @@ Apache Spark
 
 Apache Spark users can use `SynapseML`_ for machine learning workflows with LightGBM. This project is not maintained by LightGBM's maintainers.
 
-See `this SynapseML example`_ and `the SynapseML documentation`_ for additional information on using LightGBM on Spark.
+See `this SynapseML example`_ for additional information on using LightGBM on Spark.
 
 .. note::
 
@@ -489,8 +489,6 @@ See `the mars documentation`_ for usage examples.
 
 .. _the metrics functions from dask-ml: https://ml.dask.org/modules/api.html#dask-ml-metrics-metrics
 
-.. _the SynapseML Documentation: https://github.com/microsoft/SynapseML/blob/master/docs/lightgbm.md
-
 .. _these Dask examples: https://github.com/microsoft/lightgbm/tree/master/examples/python-guide/dask
 
 .. _Kubeflow Fairing: https://www.kubeflow.org/docs/components/fairing/fairing-overview