doc: fix missing iptables rul for network-for-clones.md

[huang-jl]

Existing setup in network-for-clones.md about setup egress connectivity cannot work.

Changes

Update the docs/snapshotting/network-for-clones.md to be consistent with tests/framework/microvm_helpers.py.

Reason

There lacks one iptable configuration about setup egress connectivity. As shown in

firecracker/tests/framework/microvm_helpers.py

Line 190 in 7dfe276

f"iptables -t nat -A POSTROUTING -s {veth_net} -o {upstream_dev} -j MASQUERADE"

It should add a MASQUERADE rule inside the netns. After that, the host iptable rules of source ip 10.0.0.0/30 can work. Without that, the host can only see packet of source ip (e.g., 192.168.241.1/29) from guest VM.

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PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
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  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
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---

• This functionality cannot be added in rust-vmm.

[zulinx86]

@huang-jl it looks good to me, but the style check failed.
https://buildkite.com/firecracker/firecracker-pr/builds/14209#019841ae-cd69-4617-ae6c-4638f5fc8d7f/58-280

If you'd like to have a link in commit message, please do as follows:

blah blah blah [1], blah blah

[1]: https://github.com/firecracker-microvm/firecracker/blob/7dfe2765849c2444a22defa11be8641508c5ce5c/tests/framework/microvm_helpers.py#L190
Signed-off-by: blah blah blah

As the PR check says, you can check it using tools/devtool checkstyle.

[huang-jl]

@zulinx86 Thanks for your instruction. I have update the commit message and update as @pb8o pointed out. I have test the integration_tests/style locally, and pass the previously failed gitlint test.