Update install procedure

As describe here : https://golang.org/doc/go-get-install-deprecation, go get no longer works without a go module.
Signed-off-by: Sugate <[email protected]>

Author: Joffref

content/docs/cnitool.md

@@ -27,8 +27,8 @@ add or remove an interface in an already-created network namespace.
 First, install cnitool:
 
 ```bash
-go get github.com/containernetworking/cni
-go install github.com/containernetworking/cni/cnitool
+go install github.com/containernetworking/cni@latest
+go install github.com/containernetworking/cni/cnitool@latest
 ```
 
 Then, check out and build the plugins. All commands should be run from this directory.