luci-proto-openfortivpn: Correct server certificate SHA label

According to the documentation[1], the SHA256 hash is used for the
trusted-cert option, not SHA1.

[1] https://github.com/adrienverge/openfortivpn/blob/master/doc/openfortivpn.1.in

Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>