graphs.pl: Fixes bug13838 - additional file name corrections for collectd-5.x

graphs.pl: Fixes bug13838 - additional file name corrections for collectd-5.x

- Two locations in graphs.pl were missed with filename changes coming from the collectd
   update.
- These result in missing graph content for the openvpn road warrior graphs.
- Tested out on my production IPFire system. Making the changes 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>