l2tpv3: fix missing "," within default_config_data
l2tpv3: fix missing "," within default_config_data

Commit 0e19d622a0410 ("bridge: T2232: move helper to vyos.validate") added a
new key to the dictionary but missed out the required "," at the end.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x