wg(4): Fix an example in the manual page

The example in the manual page of wg(4) for connecting to a
peer was missing the 'public-key' ifconfig(8) keyword and for the
addressed peer the port must be specified.

PR: 253866
Reported by: Sergey Akhmatov <sergey at akhmatov dot ru>
Reviewed by: debdrup
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29115