minor: tools/generate_units_documentation.py: fix regex to search for versions

Thomas Morinrequested to merge
improve-units-doc-gen into main

While working on !2304 (closed) I had to tweak tools/generate_units_documentation.py to avoid having it crash on the unusual 0.3.0-rc tag.

!2304 (closed) isn't worth pursuing now that !2328 (closed) is around, but I would like still to not lose this small fix.

Edited by Thomas Morin

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