Fix misleading commit message when updating documentation repository

[francesco-ballarin]

Noticed this while I was investigating whether to prepare an equivalent of FEniCS/basix#751 for dolfinx. At first glance I couldn't understand why there wasn't a push of the python part of the documentation: there was, but it was stored alongside the cpp in a commit message starting with C++.

[francesco-ballarin]

For reference, the reason why this doesn't need a change similar to the one in FEniCS/basix#751 is because of/thanks to the use of * in

      - name: Copy documentation into repository
        ...
        run: |
          ...
          cp -r ../python/doc/build/html/* dolfinx/${{ env.VERSION_NAME }}/python