Update version in cni tutorial

Update the cniVersion used in the CNI tutorial so that it works with
containerd 1.2. Containerd 1.2 includes a version of the cri plugin
(release/1.2) that, in turn, includes a version of the
cni library (0.6.0) that only supports up to 0.3.1.
https://github.com/containernetworking/cni/blob/v0.6.0/pkg/version/version.go#L38

PiperOrigin-RevId: 329837188

g3doc/user_guide/tutorials/cni.md

@@ -47,7 +47,7 @@ sudo mkdir -p /etc/cni/net.d
 
 sudo sh -c 'cat > /etc/cni/net.d/10-bridge.conf << EOF
 {
-  "cniVersion": "0.4.0",
+  "cniVersion": "0.3.1",
   "name": "mynet",
   "type": "bridge",
   "bridge": "cni0",
@@ -65,7 +65,7 @@ EOF'
 
 sudo sh -c 'cat > /etc/cni/net.d/99-loopback.conf << EOF
 {
-  "cniVersion": "0.4.0",
+  "cniVersion": "0.3.1",
   "name": "lo",
   "type": "loopback"
 }