tor-socksproto: fix "check_doc_features" warning

The fix was small so I just made an MR for it. It would be nice if the "check_doc_features" script gave a better info string than just '{\n'.

$ ./maint/check_doc_features
[...]
processing tor-socksproto
feature but no cfg_attr(docsrs): ('{\n', 'any(feature = "proxy-handshake", feature = "client-handshake")')
Found error, exiting