Fix \circ everywhere

[hmemcpy]

Currently the symbol for white bullet ◦ is used everywhere instead of ring operator ∘, because it won't render properly in PDF without using \ensuremath.

[Blaisorblade]

(First, thanks for this project, it's great!)

FWIW, $\circ$ would work as well as \ensuremath{\circ}. \ensuremath works both in math and non-math mode.
But using \ensuremath is often discouraged, since those modes need be distinguished anyway by the author: they obey completely different rules regarding spacing and so on.

(Sorry for the LaTeX lecture you didn't ask for).

[hmemcpy]

The LaTeX lecture is great and very welcomed! First time ever doing something in LaTeX, so I learn quickly :)) And yeah, I've already started employing more $..$ etc!
The initial thing was easier, because that's what pandoc produced out of the original WordPress html, so I decided to go with it!