fix(docs): align k8s version pre-req to v1.15
This PR ensures the minimum Kubernetes version requirement is
consistently documented to be v1.15.0 since our CI cluster is v1.15.11
and our documentation should align with the `osm check --pre-install`
command.
## System Requirements
- Go version 1.14 or higher
-- Kubectl version 1.16 or higher
+- Kubectl version 1.15 or higher
- Docker CLI
- on a Debian based GNU/Linux system: `sudo apt-get install docker`
- on a macOS use `brew install docker` or alternatively visit [Docker for Mac](https://docs.docker.com/docker-for-mac/install/)
