change in the html to fix layout issue (tested using firefox's responsive design view)

for https://github.com/swcarpentry/bc/issues/601
Keep link of lessons inside lesson div
Merge pull request #1 from r-gaia-cs/fix-layout-in-lessons-v5v4

Keep link of lessons inside lesson div
Merge pull request #553 from twitwi/fix-layout-in-lessons-v5v4

 change in the html to fix layout issue