minor revisions to open science lessons for clarity

migration of pull request swcarpentry/bc#919

@chinchliff could you please review and provide feedback on these changes?
Since a large number of our audience are academic I'd put the free academic repositories first. And I'm not aware of free repositories for open-source projects? I know Bitbucket offers limited-collaborator private repos to everyone, but they don't have to be open-source.

Suggested rewrite: "GitHub and Bitbucket offer free private repositories to academics, and more capabilities are often available via paid plans."