doc: Improve article on HTTPS
doc: Improve article on HTTPS

 * Adjust header to match _page_title
 * Add instructions on deletion of CSRs and setting permissions
 * Simplify some path expressions and commands
 * Consqeuently use ~ instead of ${HOME}
 * Precise formulation ('key' vs. 'public key')
 * Fix wrong indentation of output of `openssl req`
 * Use dash ('--') instead of minus ('-')

Remark on permissions:

It's not a problem to `chmod 0400` the private keys, because the
Docker daemon runs as root (can read the file anyway) and the Docker
client runs as user.

Signed-off-by: Lorenz Leutgeb <a8273af8124ce074690e3be166be65599e286b3f@gmail.com>

josemonsalve2/docker,AdamOssenford/docker-pi,ClusterHQ/docker-plugins,carlye566/docker_eric,carlye566/docker_eric,josemonsalve2/docker,AdamOssenford/docker-pi,seanmcdaniel/docker,seanmcdaniel/docker,ClusterHQ/docker-plugins,carlye566/docker_eric,seanmcdaniel/docker,AdamOssenford/docker-pi,ClusterHQ/docker-plugins,josemonsalve2/docker