constexpr bug fix in example/cmesh/t8_cmesh_geometry_examples.cxx

[dutkalex]

Describe your changes here:

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[holke]

Thank you for the quick PR. We will review it.

Could you please add a file to the doc/ folder stating that you put your contributions under the BSD.
Similar to the existing files, such as https://github.com/DLR-AMR/t8code/blob/main/doc/author_ackerschott.txt

[holke]

Looks good, ready to merge when the tests pass.

👍