Fix division syntax in doc comments

[Wilfred]

mod is a keyword in Rust, and since we're talking about remainders we should be using division syntax here.

[rustbot]

r? @joboet

rustbot has assigned @joboet.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[hkBst]

After doing the division (which this syntax suggests), there is no possibility of finding the remainder anymore, but perhaps you meant something like the following:

Suggested change

	/// Calculates the least nonnegative remainder of `self / rhs`.
	/// Calculates the least nonnegative remainder of `self` when divided by `rhs`.