Fix vol2slice

[kerautret]

PR Description

small fix of issue #428, thanks @dcoeurjo

Checklist

• Doxygen documentation of the code completed (classes, methods, types, members...).
• Main tool doxygen documentation (following existing documentation of DGtalTools documentation.
• Check if it follows the tools structure described in CONTRIBUTING.md
• New entry in the ChangeLog.md added.
• Update the readme with potentially a screenshot of the tools if it applies.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).
• All continuous integration tests pass (Github Actions & appveyor).

[kerautret]

ping @dcoeurjo is it fine to merge ?

[dcoeurjo]

👌 thx