OpenVPN: fix deprecated genkey usage

Avoid the following OpenVPN deprecation notice while generating the key:

    DEPRECATED OPTION: The option --secret is deprecated.
    WARNING: Using --genkey --secret filename is DEPRECATED.  Use --genkey secret filename instead.

In short:

    #Bad:  'sudo openvpn --genkey --secret ta.key'
    #Good: 'sudo openvpn --genkey   secret ta.key'

Ref #216

Author: valerio-bozzolan

how-to/security/install-openvpn.md

@@ -135,7 +135,7 @@ tls-auth ./easy-rsa/ta.key 0
 Complete this set with a TLS Authentication (TA) key in `etc/openvpn` for `tls-auth` like this:
 
 ```bash
-sudo openvpn --genkey --secret ta.key
+sudo openvpn --genkey secret ta.key
 ```
 
 Edit `/etc/sysctl.conf` and uncomment the following line to enable IP forwarding: