netovpnrw.cgi: Fixes bug13838 - additional file name correction for collectd-5.x

netovpnrw.cgi: Fixes bug13838 - additional file name correction for collectd-5.x

- One location in netovpnrw.cgi was missed with a filename change coming from the collectd
   update.
- This resulted in missing graph content for the openvpn road warrior graphs.
- Tested out on my production IPFire system. Making the change resulted in the grahs
   being visible again.

Fixes: Bug13838
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>