Fixed problem with range_overlap tests in ipynb file
Fixed problem with range_overlap tests in .md file
Updated an error message
Removed reference to the reverse method in ipynb file
Removed reference to the reverse method in md file
Edited wording of reverse challenge in ipynb file
Edited wording of reverse challenge in md file
Put 3 spaces after bullet points
Merge pull request #526 from DamienIrving/lesson_fixes

Some minor fixes to challenges in some lessons