docs: fix a broken link in README
docs: fix a broken link in README

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

j-kato/gobgp,a16/gobgp,ishidawataru/gobgp,ishidawataru/gobgp,j-kato/gobgp,j-kato/gobgp,osrg/gobgp,Dipsingh/gobgp,h-naoto/gobgp,YujiOshima/gobgp,tamihiro/gobgp,postfix/gobgp,Dipsingh/gobgp,fujita/gobgp,tamihiro/gobgp,YujiOshima/gobgp,fujita/gobgp,fujita/gobgp,a16/gobgp,ramrunner/gobgp,postfix/gobgp,YujiOshima/gobgp,a16/gobgp,ishidawataru/gobgp,a16/gobgp,a16/gobgp,ramrunner/gobgp,h-naoto/gobgp,ramrunner/gobgp,fujita/gobgp,osrg/gobgp,Dipsingh/gobgp,tamihiro/gobgp,tamihiro/gobgp,osrg/gobgp,postfix/gobgp,a16/gobgp,h-naoto/gobgp,a16/gobgp,osrg/gobgp