Update Calico to lowercase and fix broken calico link in README

Signed-off-by: Xieql <xieqianglong@huawei.com>

README.md

@@ -227,7 +227,7 @@ You can choose among ten network plugins. (default: `calico`, except Vagrant use
 
 - [flannel](docs/flannel.md): gre/vxlan (layer 2) networking.
 
-- [Calico](https://docs.projectcalico.org/latest/introduction/) is a networking and network policy provider. Calico supports a flexible set of networking options
+- [calico](https://docs.tigera.io/calico/latest/about/) is a networking and network policy provider. Calico supports a flexible set of networking options
     designed to give you the most efficient networking across a range of situations, including non-overlay
     and overlay networks, with or without BGP. Calico uses the same engine to enforce network policy for hosts,
     pods, and (if using Istio and Envoy) applications at the service mesh layer.