fix unit conversion in openmc.deplete.Results.get_mass

[nplinden]

Description

The openmc.deplete.Results class provides a get_mass method that can provides mass values in g (default), g/cm3, and kg.
To convert from g to kg, the mass is multiplied by 1_000, which convert to mg instead.

Fixes

I changed to * operator to the / operator for correct conversion.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[shimwell]

Wow nice find @nplinden much appreciated. Also GitHub is telling me this is your first PR, welcome aboard.