Update tutorial.md for grammatical changes

Update tutorial.md to improve grammatical parallelism in features list. Also added periods to follow first item's syntax.

BEFORE: "Live updates: as other users comment we'll pop them into the comment view in real time"
AFTER: "Live updates: other users' comments are popped into the comment view in real time."

BEFORE: "Markdown formatting: users can use Markdown to format their text"
AFTER: "Markdown formatting: users can use Markdown to format their text."
Merge pull request #2718 from alextsg/patch-1

Update tutorial.md for grammatical changes