Change . for : in debian's postinst

[f-galland]

Description

This PR fixes an error in chown's syntax within debian's postinst script that issues a warning on packages installation.

Issues Resolved

Resolves #244

Check List

• [x] New functionality includes testing.
  • [x] All tests pass
• [ ] New functionality has been documented.
  • [ ] New functionality has javadoc added
• [ ] Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• [x] Commits are signed per the DCO using --signoff
• [ ] Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• [ ] Public documentation issue/PR created

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