title: Pyspark dataframe write to single json file with specific name
body: <p>I have a dataframe which I want to write it as single json file with a specific name. I tried below</p>
  
  <pre><code>df2 = df1.select(df1.col1,df1.col2)
  df2.write.format('json').save('/path/file_name.json') # didnt work, writing in folder 'file_name.json' and files with part-XXX
  df2.toJSON().saveAsTextFile('/path/file_name.json')  # didnt work, writing in folder 'file_name.json' and files with part-XXX
  </code></pre>
  
  <p>Appreciate if some one can provide a solution.</p>