Update docker-openvpn@.service (docker.socket)

Since version 1.12, Docker removed `docker.socket` <https://docs.docker.com/release-notes/docker-engine/>

The problem was already present on RPM systems (e.g. RHEL, Fedora, CentOS), and now also seems to affect Debian 9 (Stretch).

This change replaces `docker.socket` by `docker.service`.

It solves this problem:

```
> systemctl start docker-openvpn@.service
Failed to start docker-openvpn@.service: Unit docker.socket not found.
```

Related issues:

 - <#280>
 - <#286>

Author: kunnix

init/docker-openvpn@.service

@@ -33,8 +33,8 @@
 [Unit]
 Description=OpenVPN Docker Container
 Documentation=https://github.com/kylemanna/docker-openvpn
-After=network.target docker.socket
-Requires=docker.socket
+After=network.target docker.service
+Requires=docker.service
 
 [Service]
 RestartSec=10