docs: fix output example for docker system prune

The example shows that the `--volumes` option is used, which in current
versions of docker only removes "anonymous" volumes, but preserves named
volume:

    $ docker system prune -a --volumes
    ...
            - all anonymous volumes not used by at least one container
    ...

But the example output showed that a named volume ("named-vol") was
deleted;

    Deleted Volumes:
    named-vol

Co-authored-by: Roberto Villarreal <rrjjvv@yahoo.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/system_prune.md

@@ -72,7 +72,8 @@ my-network-a
 my-network-b
 
 Deleted Volumes:
-named-vol
+1e31bcd425e913d9f65ec0c3841e9c4ebb543aead2a1cfe0d95a7c5e88bb5026
+6a6ab3d6b8d740a1c1d4dbe36a9c5f043dd4bac5f78abfa7d1f2ae5789fe60b0
 
 Deleted Images:
 untagged: my-curl:latest