luci-app-openvpn: fix advanced mode page switcher translation

Translation of page titles in the advanced configuration mode is done
in such a way that the strings to be translated are not visible for
'i18n-scan.pl' script. This commit fixes this and makes page titles
visible for the 'i18n-scan.pl'.

Signed-off-by: Anton Kikin <a.kikin@tano-systems.com>