Updating the Atom download link

For ease of those about to join us, I think the Atom link in the Alpha Course page should send users to either:

https://atom.io/download/mac

or just

Right now it points to https://atom.io/%20target= which returns a 404 error.

Hi David,

Thank you so much for reporting on the broken links in the install requirements reading. These links have been updated!

Keep up the great work.