[MINOR] fix RegressionEvaluator doc

`make clean html` under `python/doc` returns
~~~
/Users/meng/src/spark/python/pyspark/ml/evaluation.py:docstring of pyspark.ml.evaluation.RegressionEvaluator.setParams:3: WARNING: Definition list ends without a blank line; unexpected unindent.
~~~

harsha2010

Author: Xiangrui Meng <meng@databricks.com>

Closes #6469 from mengxr/fix-regression-evaluator-doc and squashes the following commits:

91e2dad [Xiangrui Meng] fix RegressionEvaluator doc

python/pyspark/ml/evaluation.py

@@ -205,7 +205,7 @@ class RegressionEvaluator(JavaEvaluator, HasLabelCol, HasPredictionCol):
     def setParams(self, predictionCol="prediction", labelCol="label",
                   metricName="rmse"):
         """
-        setParams(self, predictionCol="prediction", labelCol="label",
+        setParams(self, predictionCol="prediction", labelCol="label", \
                   metricName="rmse")
         Sets params for regression evaluator.
         """