Fix path of generated openAPI

[Shunpoco]

Thank you for your fabulous hands-on!

What

Fix the path of generated openAPI code in ./hack/openapi.sh.

Problem

When I tried to execute make openapi, I caught an error:

$ make openapi
./hack/openapi.sh
make[1]: Entering directory '<my_path>/mini-kube-scheduler/submodules/kubernetes'
make[2]: Entering directory '<my_path>/mini-kube-scheduler/submodules/kubernetes'
<...omit...>
make[1]: Leaving directory '<my_path>/mini-kube-scheduler/submodules/kubernetes'
cp: cannot create regular file '../k8sapiserver/openapi/zz_generated.openapi.go': No such file or directory
make: *** [Makefile:16: openapi] Error 1

We should specify ../../k8sapiserver/openapi/zz_... instead of ../k8sapiserver/openapi/zz_... because in openapi.sh, the working directory is changed to ./submodules/kubernetes.