Fix JsonlWriter / ParquetWriter output directory typo

[jrbourbeau]

These both raise errors like this

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[2], line 1
----> 1 writer = JsonlWriter(output_dir="./cc_output")

TypeError: JsonlWriter.__init__() got an unexpected keyword argument 'output_dir'

This PR updates these snippets to pass the output directory as a positional argument instead

[jrbourbeau]

Thanks @praateekmahajan!