readme: replace pingcap links with repo links (#3927)
readme: replace pingcap links with repo links (#3927)

Signed-off-by: lilin90 <c684465c17314189c18707959627e2ec4fdec2d0@pingcap.com>
BusyJay/tikv,BusyJay/tikv,pingcap/tikv,BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv,pingcap/tikv