[MINOR] [ML] fix doc warnings

[mengxr]

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])".

[SparkQA]

Test build #44012 has finished for PR 9188 at commit 7f123f0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class BinaryClassificationEvaluator @Since(\"1.4.0\") (@Since(\"1.4.0\") override val uid: String)\n * class MulticlassClassificationEvaluator @Since(\"1.5.0\") (@Since(\"1.5.0\") override val uid: String)\n * final class RegressionEvaluator @Since(\"1.4.0\") (@Since(\"1.4.0\") override val uid: String)\n * abstract class ColumnarIterator extends Iterator[InternalRow]\n * class SpecificColumnarIterator extends $\n

[holdenk]

LGTM, sorry about that.

[mengxr]

NP. Merged into master.