tor.cgi: Fixes deprecated tor option 'ExitNode' to 'ExitNodes'

tor.cgi: Fixes deprecated tor option 'ExitNode' to 'ExitNodes'

If fingerprints in the Exit Node section are in usage, tor.cgi prints the
deprecated option 'ExitNode' into torrc which leads to the following warning
"The abbreviation ‘ExitNode’ is deprecated. Please use ‘ExitNodes’ instead".
Fix has been found and tested in the community for reference please see -->
https://community.ipfire.org/t/the-abbreviation-exitnode-is-deprecated-please-use-exitnodes-instead/10582/10

Signed-off-by: Erik Kapfer <erik.kapfer@ipfire.org>