docs: fix inconsistencies in getting-started.md
docs: fix inconsistencies in getting-started.md

Fix some inconsistencies found in getting-started.md:
- there are some leftover references to Virtual Machines that actually
  mean Virtual Machine Instances.
- add a reminder to look also in non-default namespaces when checking
  control plane pods.
- fix typo(s).

Signed-off-by: Francesco Romani <c7100b580e6017ef67482ace46578cdd2d2e0dfe@redhat.com>

kubevirt/kubevirt,davidvossel/kubevirt,davidvossel/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt,stu-gott/kubevirt,stu-gott/kubevirt,kubevirt/kubevirt,davidvossel/kubevirt,davidvossel/kubevirt,stu-gott/kubevirt,stu-gott/kubevirt