Fixes in Arrow docs (#2023)

* Fix type signature of Arrow.second in doc

* Fix signature of Arrow.split in doc

Author: Jasper-M

docs/src/main/tut/typeclasses/arrow.md

@@ -12,8 +12,8 @@ scaladoc: "#cats.arrow.Arrow"
 Having an `Arrow` instance for a type constructor `F[_, _]` means that an `F[_, _]` can be composed and combined with other `F[_, _]`s. You will be able to do things like:
 - Lifting a function `ab: A => B` into arrow `F[A, B]` with `Arrow[F].lift(ab)`. If `F` is `Function1` then `A => B` is the same as `F[A, B]` so `lift` is just the identity function.
 - Composing `fab: F[A, B]` and `fbc: F[B, C]` into `fac: F[A, C]` with `Arrow[F].compose(fbc, fab)`, or `fab >>> fbc`. If `F` is `Function1` then `>>>` becomes an alias for `andThen`.
-- Taking two arrows `fab: F[A, B]` and `fbc: F[B, C]` and combining them into `F[(A, C) => (B, D)]` with `fab.split(fbc)` or `fab *** fbc`. The resulting arrow takes two inputs and processes them with two arrows, one for each input.
-- Taking an arrow `fab: F[A, B]` and turning it into `F[(A, C), (B, C)]` with `fab.first`. The resulting arrow takes two inputs, processes the first input and leaves the second input as it is. A similar method, `fab.second`, turns `F[A, B]` into `F[(C, A), (B, A)]`.
+- Taking two arrows `fab: F[A, B]` and `fcd: F[C, D]` and combining them into `F[(A, C) => (B, D)]` with `fab.split(fcd)` or `fab *** fcd`. The resulting arrow takes two inputs and processes them with two arrows, one for each input.
+- Taking an arrow `fab: F[A, B]` and turning it into `F[(A, C), (B, C)]` with `fab.first`. The resulting arrow takes two inputs, processes the first input and leaves the second input as it is. A similar method, `fab.second`, turns `F[A, B]` into `F[(C, A), (C, B)]`.
 
 ## Examples