Minor improvements to contributing guide

[dstansby]

• Highlight code with bash highlighting
• Fix highlighting in markdown code block
• Remove leading "$" in code blocks to make it easier to copy/paste commands
• Add quotes around [docs] so that the command works as intended in zsh.
• Remove open command in doc building section, as this command is only available on macOS.

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)