Update macOS instructions in Numo docs

[mkasberg]

In #198, I added support for using Numo::Linalg as the linear algebra backend for classifier-reborn. At that time, I updated the docs with instructions for installing Numo, but the macOS docs were a little vague because I hadn't tested them myself. Since then, I've been able to verify the instructions on macOS and clarify a few steps. So this commit updates the docs for installing Numo on macOS. The gem installation arguments I'm using come from the Numo docs.