Fix 'occured' -> 'occurred' typos in vmm doc comments

[SAY-5]

Fix 3 instances of occured → occurred:

• src/vmm/src/mmds/ns.rs line 144: Doc comment on return value of is_mmds_frame
• src/vmm/src/snapshot/mod.rs line 60: Error display string (#[error(...)]) — surfaced in error output
• src/vmm/src/devices/virtio/vsock/unix/muxer.rs line 285: Doc comment on notify

Comment/error-string-only change; no logic change.

PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md. N/A, doc-only change.
• All added/changed functionality is tested. N/A, doc-only change.

[zulinx86]

Hi @SAY-5 , thank you for fixing them!

To make our CI pass, you need to fill in the commit messages. Also could you combine them all into a single commit as it fixes the same typo.

Thanks!

[SAY-5]

Closing — typo-only PR. Multiple maintainers across the OSS ecosystem have flagged my recent typo-sweep PRs as AI-generated spam (notably hashicorp/nomad#27855, hashicorp/hcl#794, argo-cd reviewers). I should have caught this pattern sooner. Apologies for the noise.