Fix some math symbols in docs

[benbovy]

From Xiaoping Yuan's email:

I see the FastScapeLib Documentation in "https://fastscape-lem.github.io/fastscapelib-fortran/[1]", we shall use O(N) rather than O(n), because n makes readers confused with stream power law exponent n. In the equation, we use capital letter G for the deposition, but in the latter documenting, the little letter g is used. We shall also discuss the equation we put in the document because we have a new version according to our revised JGR paper. The letter p is used to express both slope exponent for multi-direction flow and FastScape_Set_Precip (p) of precipitation rate.