[Bug] update dragonfly doc to use proxy mode

[imeoer]

Problem Description

We have removed mirror mode from the nydus doc here.

Expected Behavior

Let's update it to proxy mode.

Actual Behavior

Request dragonfly by mirror mode.

Environment Details

• Nydus-snapshotter version:
• Nydus version:
• Container runtime:
• Operating System:
• Kernel version:

Additional Information

No response

Are you willing to submit PR?

• [ ] Yes I am willing to submit a PR!

[imeoer]

related issue: https://github.com/containerd/nydus-snapshotter/issues/647