Add rc_context to customizing tutorial

[rcomer]

PR Summary

rc_context is really useful and I think it deserves a mention in the "Customizing" tutorial. While I was checking which options to use, I realised the comment in the linestyles example was wrong, so fixed it while I was there.

PR Checklist

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [N/A] New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[story645]

Thanks for the fix and the addition to the tutorial!