typo in "Management of K8s clusters" section, option "--k8s_nets" that shall have been "--k8s-nets"

Hence, in other to support this case, OSM's NBI provides a mechanism to be informed about existing K8s clusters. The corresponding CLI client command is the following:

```bash

osm k8scluster-add --creds <credentials_file.yaml> --vim <VIM_target> --version <ver=version> [--namespace "namespace_name"] [--cni "cni_plugin"] --k8s_nets '{(k8s_net1:vim_network1) [,(k8s_net2:vim_network2) ...]}' <name>

osm k8scluster-add --creds <credentials_file.yaml> --vim <VIM_target> --version <ver=version> [--namespace "namespace_name"] [--cni "cni_plugin"] --k8s-nets '{(k8s_net1:vim_network1) [,(k8s_net2:vim_network2) ...]}' <name>

```

Where:

- `namespace`. By default, it will use `kube-system` for this operation.

- `VIM_target`: The VIM where the cluster would reside or be attached (the case of bare metal clusters is simply considered a particular case here).

- `ver`: K8s version

- `k8s_nets`: list of VIM networks where the cluster is accessible via L3 routing, in (key,value) format, where:

- `k8s-nets`: list of VIM networks where the cluster is accessible via L3 routing, in (key,value) format, where:

  - The _key_ will be used to refer to a given cluster's network (e.g. "mgmt", "external", etc.)

  - The _value_ will be used to refer to a VIM network that provides L3 access to that cluster network.

- Optionally: