Correct parquet nullability documentation

Parquet files appear to have nullability info when being written, not being read.  Some test code:

case class NullTest(bo: Boolean, opbol: Option[Boolean])
val testDf = spark.createDataFrame(Seq(NullTest(true, Some(false))))

defined class NullTest
testDf: org.apache.spark.sql.DataFrame = [bo: boolean, opbol: boolean]

testDf.write.parquet("s3://asana-stats/tmp_dima/parquet_check_schema")

spark.read.parquet("s3://asana-stats/tmp_dima/parquet_check_schema/part-00000-b1bf4a19-d9fe-4ece-a2b4-9bbceb490857-c000.snappy.parquet4").printSchema()
root
 |-- bo: boolean (nullable = true)
 |-- opbol: boolean (nullable = true)


Meanwhile, the parquet file formed does have nullable info:

[]batch@prod-report000:/tmp/dimakamalov-batch$ aws s3 ls s3://asana-stats/tmp_dima/parquet_check_schema/
2018-10-17 21:03:52          0 _SUCCESS
2018-10-17 21:03:50        504 part-00000-b1bf4a19-d9fe-4ece-a2b4-9bbceb490857-c000.snappy.parquet
[]batch@prod-report000:/tmp/dimakamalov-batch$ aws s3 cp s3://asana-stats/tmp_dima/parquet_check_schema/part-00000-b1bf4a19-d9fe-4ece-a2b4-9bbceb490857-c000.snappy.parquet .
download: s3://asana-stats/tmp_dima/parquet_check_schema/part-00000-b1bf4a19-d9fe-4ece-a2b4-9bbceb490857-c000.snappy.parquet to ./part-00000-b1bf4a19-d9fe-4ece-a2b4-9bbceb490857-c000.snappy.parquet
[]batch@prod-report000:/tmp/dimakamalov-batch$ java -jar parquet-tools-1.8.2.jar schema part-00000-b1bf4a19-d9fe-4ece-a2b4-9bbceb490857-c000.snappy.parquet 
message spark_schema {
  required boolean bo;
  optional boolean opbol;
}

Author: dima-asana

docs/sql-programming-guide.md

@@ -706,7 +706,7 @@ data across a fixed number of buckets and can be used when a number of unique va
 
 [Parquet](http://parquet.io) is a columnar format that is supported by many other data processing systems.
 Spark SQL provides support for both reading and writing Parquet files that automatically preserves the schema
-of the original data. When writing Parquet files, all columns are automatically converted to be nullable for
+of the original data. When reading Parquet files, all columns are automatically converted to be nullable for
 compatibility reasons.
 
 ### Loading Data Programmatically