[MINOR][ML] fix doc warnings

Without an empty line, sphinx will treat doctest as docstring. holdenk

~~~
/Users/meng/src/spark/python/pyspark/ml/feature.py:docstring of pyspark.ml.feature.CountVectorizer:3: ERROR: Undefined substitution referenced: "label|raw |vectors | +-----+---------------+-------------------------+ |0 |[a, b, c] |(3,[0,1,2],[1.0,1.0,1.0])".
/Users/meng/src/spark/python/pyspark/ml/feature.py:docstring of pyspark.ml.feature.CountVectorizer:3: ERROR: Undefined substitution referenced: "1 |[a, b, b, c, a]|(3,[0,1,2],[2.0,2.0,1.0])".
~~~

Author: Xiangrui Meng <meng@databricks.com>

Closes #9188 from mengxr/py-count-vec-doc-fix.

python/pyspark/ml/feature.py

@@ -178,6 +178,7 @@ class CountVectorizer(JavaEstimator, HasInputCol, HasOutputCol):
     .. note:: Experimental
 
     Extracts a vocabulary from document collections and generates a :py:attr:`CountVectorizerModel`.
+
     >>> df = sqlContext.createDataFrame(
     ...    [(0, ["a", "b", "c"]), (1, ["a", "b", "b", "c", "a"])],
     ...    ["label", "raw"])