Minor edits to the core concepts page

[narenst]

Switched from third person to second person. This was the only place third person was used, so it was a little off to read.

Also the overview titles seemed superflous, so I cut them.

Let me know if you have already thought about them, just figured it was easier to make a PR to discuss about.

[mckayward]

Yeah I thought about the overview titles. Without them I thought it looked a little weird, or like a user might be less likely to read on. But I didn't feel strongly about it, so I am open to scrapping them. You're right that they don't really do much

[narenst]

Cool, the changes look good. This can be merged!