Update certificates.md
Update certificates.md

`openssl req -new -x509 -text -key client.key -out client.cert` creates a self-sign certificate but not a certificate request.

Signed-off-by: Wei-Ting Kuo <21f82bcb97ea3f52e51acfd0ed231f2692b70330@gmail.com>

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