doc: fix package import path in sample code

fix package import path that was changed as part of v2 in sample code
snippets.

Signed-off-by: Akhil Mohan <akhilerm@gmail.com>

README.md

@@ -109,8 +109,8 @@ import (
   "context"
 
   containerd "github.com/containerd/containerd/v2/client"
-  "github.com/containerd/containerd/v2/cio"
-  "github.com/containerd/containerd/v2/namespaces"
+  "github.com/containerd/containerd/v2/pkg/cio"
+  "github.com/containerd/containerd/v2/pkg/namespaces"
 )