doc: fix default runtime on k8s_pool (#873)

Author: jtherin

docs/resources/k8s_pool.md

@@ -28,7 +28,7 @@ resource "scaleway_k8s_pool" "bill" {
   max_size           = 10
   autoscaling        = true
   autohealing        = true
-  container_runtime  = "docker"
+  container_runtime  = "containerd"
   placement_group_id = "1267e3fd-a51c-49ed-ad12-857092ee3a3d"
 }
 ```
@@ -63,7 +63,7 @@ The following arguments are supported:
 
 - `autohealing` - (Defaults to `false`) Enables the autohealing feature for this pool.
 
-- `container_runtime` - (Defaults to `docker`) The container runtime of the pool.
+- `container_runtime` - (Defaults to `containerd`) The container runtime of the pool.
 ~> **Important:** Updates to this field will recreate a new resource.
 
 - `kubelet_args` - (Optional) The Kubelet arguments to be used by this pool