[Hotfix Main]: [HOTFIX] examples: changed file name

[github-actions]

Hotfix of PR #1753 (#1753) to the main branch.
Hey @marcvivesmassana, please review this hotfix PR created from your original PR.

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Note

Low Risk
Low risk: a one-line import fix to match a renamed examples module; impact is limited to package importability of the DARPA_SUBOFF example.

Overview
Updates flow360/examples/__init__.py to import DARPA_SUBOFF from DARPA_SUBOFF.py instead of the old DARPA module name, keeping the example available via flow360.examples after the filename change.

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