Update radiation_models.rst

[tpoiii]

Corrected spherical shielding description for SHIELDOSE2: it is not illuminated from all directions, only one hemisphere.

Pull Request Checklist

New code is easier to review, integrate and maintain if it's
clear, commented, and consistent with the style of the rest
of the IRBEM code.
Please try to follow as much of this checklist as you can for
your PR. If you can't hit everything, or don't know how to,
then submit the PR and the rest can be discussed during review.
Some additional suggestions are given below.

Please also see our Code of Conduct so that the PRBEM community
remains welcoming and inclusive.

Go ahead and replace the text above (and this line!) with your
pull request description.

• Pull request has descriptive title
• Pull request gives overview of changes
• New code has inline comments where necessary
• Appropriate documentation has been written
• Relevant issues are linked to (e.g. See issue # or Closes #)

[AntoineBrunet]

The PR looks fine to me. I trust Paul on this, and it seems to match the output variable description which appears later:

SolDose: dose profile for solar protons (rads)
         1. Dose in semi-infinite aluminium medium
         2. Dose at transmission surface of finite aluminium slab shields
         3. 1/2 Dose at center of aluminium spheres