Fix checkpoint filename

On podman 5.2.5, the checkpoint archive is a zstd compressed tar file.

Author: hanwen

docs/checkpoint.md

@@ -56,14 +56,14 @@ transferring the checkpoint, it is possible to specify an output-file.
 On the source system:
 
 ```console
-$ sudo podman container checkpoint <container_id> -e /tmp/checkpoint.tar.gz
-$ scp /tmp/checkpoint.tar.gz <destination_system>:/tmp
+$ sudo podman container checkpoint <container_id> -e /tmp/checkpoint.tar.zstd
+$ scp /tmp/checkpoint.tar.zstd <destination_system>:/tmp
 ```
 
 On the destination system:
 
 ```console
-$ sudo podman container restore -i /tmp/checkpoint.tar.gz
+$ sudo podman container restore -i /tmp/checkpoint.tar.zstd
 ```
 
 After being restored, the container will answer requests again as it did before