Typos on contributions.html

[kzhang01]

I've found the following typos in contributions.html:

• students' is misspelled as studnets' on slide 2
• misformatting is misspelled as miss-formatting on slide 8
• There is an extra "If you" in the fourth bullet point on slide 8
• existing is misspelled as exiting on slide 15

[ppark3]

I've also found a couple more typos in contributions.html:

• "fix" is spelled as "fixe" on slide 2
• "documentation writing" is spelled as "documnetation writing" on slide 4

[hunjunChoi]

A minor grammatical error in the same slide deck:

• "an issues" should be "an issue" on the second paragraph of slide 8.

[hunjunChoi]

Hi @joannakl, I would like to work on the issue

[joannakl]

Go ahead @hunjunChoi