minor style fix

Author: Wayne Zhang

python/pyspark/ml/feature.py

@@ -2086,7 +2086,7 @@ class StringIndexer(JavaEstimator, HasInputCol, HasOutputCol, HasHandleInvalid,
     so the most frequent label gets index 0. The ordering behavior is controlled by
     setting :py:attr:`stringOrderType`.
 
-    >>> stringIndexer = StringIndexer(inputCol="label", outputCol="indexed", handleInvalid='error')
+    >>> stringIndexer = StringIndexer(inputCol="label", outputCol="indexed", handleInvalid="error")
     >>> model = stringIndexer.fit(stringIndDf)
     >>> td = model.transform(stringIndDf)
     >>> sorted(set([(i[0], i[1]) for i in td.select(td.id, td.indexed).collect()]),
@@ -2114,7 +2114,7 @@ class StringIndexer(JavaEstimator, HasInputCol, HasOutputCol, HasHandleInvalid,
     True
     >>> stringIndexer.getStringOrderType()
     'frequencyDesc'
-    >>> stringIndexer = StringIndexer(inputCol="label", outputCol="indexed", handleInvalid='error',
+    >>> stringIndexer = StringIndexer(inputCol="label", outputCol="indexed", handleInvalid="error",
     ...     stringOrderType="alphabetDesc")
     >>> model = stringIndexer.fit(stringIndDf)
     >>> td = model.transform(stringIndDf)