[MINOR][DOCS][PYTHON] Adding missing boolean type for replacement value in fillna

## What changes were proposed in this pull request?

Currently pyspark Dataframe.fillna API supports boolean type when we pass dict, but it is missing in documentation.

## How was this patch tested?
>>> spark.createDataFrame([Row(a=True),Row(a=None)]).fillna({"a" : True}).show()
+----+
|   a|
+----+
|true|
|true|
+----+

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Srinivasa Reddy Vundela <vsr@cloudera.com>

Closes #17688 from vundela/fillna_doc_fix.