title: pyspark to_timestamp does not include milliseconds
body: <p>I'm trying to format my timestamp column to include milliseconds without success.  How can I format my time to look like this -  <code>2019-01-04 11:09:21.152</code> ? </p>
  
  <p>I have looked at the documentation and following the SimpleDataTimeFormat , which the pyspark docs say are being used by the <code>to_timestamp</code> function. </p>
  
  <p>This is my dataframe. </p>
  
  <pre><code>+--------------------------+
  |updated_date              |
  +--------------------------+
  |2019-01-04 11:09:21.152815|
  +--------------------------+
  </code></pre>
  
  <p>I use the millisecond format without any success as below </p>
  
  <pre><code>&gt;&gt;&gt; df.select('updated_date').withColumn("updated_date_col2", 
  to_timestamp("updated_date", "YYYY-MM-dd HH:mm:ss:SSS")).show(1,False)
  +--------------------------+-------------------+
  |updated_date              |updated_date_col2  |
  +--------------------------+-------------------+
  |2019-01-04 11:09:21.152815|2019-01-04 11:09:21|
  +--------------------------+-------------------+
  </code></pre>
  
  <p>I expect <code>updated_date_col2</code> to be formatted as <code>2019-01-04 11:09:21.152</code></p>