title: how to save dataframe into csv pyspark
body: <p>I am trying to save dataframe into hdfs system.
  It gets saved as part-0000 and into multiple parts.
  I want to save it as an excel sheet or just one part file?
  How can we achieve this?</p>
  
  <p>code used so far:</p>
  
  <pre><code>  df1.write.csv('/user/gtree/tree.csv')
  </code></pre>