Error found in graphViewer.cpp

[ngophuc]

Thanks a lot for contributing to DGtalTools-contrib, before submitting your PR, please fill up the description and make sure that all checkboxes are checked.

PR Description

your description here

Checklist

• [ ] Doxygen documentation of the code completed (classes, methods, types, members...)
• [ ] 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).

[kerautret]

Thanks @ngophuc ! Sorry for the delay! Could just add you in the changelog ?

[kerautret]

Thanks @ngophuc ! Sorry for the delay! Could just add you in the changelog ?

I do it for you merging..