update the WG slack link (#5530)
update the WG slack link (#5530)

Signed-off-by: Calvin Weng <03310b1688af3ac6d84b03af03b0c4d581e884d9@pingcap.com>
BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv,pingcap/tikv,BusyJay/tikv