Revert index.jsp to original content

[jiwonhu]

index.jsp has updated file from lesson 3-20, not original file from 3-19. So it confuses the students when they follow the course. It also invalidates the purpose of lesson 3-20 because the student would already have the updated code from lesson 3-19. This reverts the file to 3-19 version but also include 3-20 content as a comment, so the students can easily update the file when they enter 3-20 lesson. Also fixed typos.