Fix some steps in README.md to install flux by helm

[at-ishikawa]

Summary

Update next steps in README.md for instruction to install flux by helm.

1. Update github URL format for https
2. Add namespace to create k8s secret

Error details

1. If we see the logs on k8s pods, next logs can be seen (USERNAME, TOKEN, and REPOSITORY are not the real values and they should be real values).

   ts=2019-10-14T05:47:42.358161033Z caller=loop.go:101 component=sync-loop err="git repo not ready: git clone --mirror: fatal: unable to access 'https://USERNAME:[email protected]:at-ishikawa/REPOSITORY.git/': URL using bad/illegal format or missing URL, full output:\n Cloning into bare repository '/tmp/flux-gitclone859872933'...\nfatal: unable to access 'https://USERNAME:[email protected]:at-ishikawa/REPOSITORY.git/': URL using bad/illegal format or missing URL\n"
   

2. If we do not add namespace in flux secret, we can get following events on the result of kubectl describe pods.

   # kubectl -n flux describe pods -l app=flux
   ## A lot of details
   Events:
     Type     Reason     Age               From                                                       Message
     ----     ------     ----              ----                                                       -------
   ## A lot of events
     Warning  Failed     1s (x9 over 94s)  kubelet, gke-cluster-preemptible-node-pool-9d547b9e-8lj1  Error: secrets "flux-git-auth" not found
   

[stefanprodan]

LGTM

Thanks @at-ishikawa