Update Parsedown.php

This solves a bug which stops referenced links formatting correctly when a space is inserted before a reference to a link right after a list.

This is caused because parsedown think's it's a secondary list rather, so it jumps to the next line.

Here is an example that shows the bug:

    + [Google][2]
    + [Youtube][1]  

      [1]: http://youtube.com
      [2]: http://google.com