Motivation

This PR includes minor documentation updates to improve consistency in formatting within docstrings in the optuna/terminator module. Specifically, single backticks have been replaced with double backticks for inline code references.

Description of the changes

• Inline code-blocks are supposed to be wrapped with double backtics.
  • https://github.com/optuna/optuna/wiki/Coding-Style-Conventions#docstrings
  • https://sublime-and-sphinx-guide.readthedocs.io/en/latest/inline.html#code-text.