Update persistent_volumes.md
Update persistent_volumes.md

In main text of this doc, "minikube" and "Minikube" both appears. 
Minikube is a proper noun. So it's better use same format in a doc.
kubernetes/minikube,kubernetes/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,warmchang/minikube,dims/minikube,dalehamel/minikube,dlorenc/minikube,warmchang/minikube,dlorenc/minikube,kubernetes/minikube,dims/minikube,warmchang/minikube,dlorenc/minikube,dlorenc/minikube,dlorenc/minikube,dalehamel/minikube,dims/minikube,aaron-prindle/minikube,warmchang/minikube,dlorenc/minikube,warmchang/minikube,kubernetes/minikube,dlorenc/minikube,kubernetes/minikube,warmchang/minikube,dlorenc/minikube,aaron-prindle/minikube,dalehamel/minikube,aaron-prindle/minikube