fix path in developer guide
Fork the [cluster-api-provider-aws repo](https://github.com/kubernetes-sigs/cluster-api-provider-aws):
```bash
-cd "$(go env GOPATH)"
+cd "$(go env GOPATH)"/src
mkdir sigs.k8s.io
cd sigs.k8s.io/
git clone git@github.com:<GITHUB USERNAME>/cluster-api-provider-aws.git
