ipv6: fix typo in ipv6_addr_to_str documentation
ipv6: fix typo in ipv6_addr_to_str documentation

It doesn't make any sense that `result_len` should specify the length of
`result_len`. Instead, `result_len` should specify the length of the
`result` parameter.

authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,OlegHahm/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT