fix docstring of ARKODE

[ranocha]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

The docstring rendering https://docs.sciml.ai/DiffEqDocs/dev/api/sundials/#Sundials.ARKODE is broken (underscores cause italic text etc.).