Update class4.html

Working with instructor during class to review slides
1) First slide on static positioning - left-aligned images so that they reflect actual normal flow with static positioning (were centered)
2) Renamed example classes for clarification; naming the class the same as the property was causing confusion with our students 
(example: .float { float: left; } 
changed to 
.float-example { float: left;} )

--general code cleanup as i was going through to improve readability