reference.md: remove old, add new things

[maxim-belkin]

1. I propose to consider adding "tab completion" to the glossary (reference.md). We talk about it in episode 2.
2. I'm not sure where in the lesson we talk about RGB, so, perhaps, we can remove it (?).

[ldko]

Adding "tab completion" to the glossary sounds fine to me.

Regarding RGB, there is content about it in https://swcarpentry.github.io/python-novice-inflammation/discuss/. However, I am not sure how much anyone is using this content as it is not particularly relevant to the rest of the lesson materials. Perhaps, that should be removed as well?