Update characteristics of default context

[straight-shoota]

Since crystal-lang/crystal#16136 the default context is Parallel but with size 1.

[ysbaddaden]

Q: shall we edit archived RFCs?

Maybe keep the original text, but add an errata callout about the actual decision acted in Crystal 1.18.0?

[straight-shoota]

Yeah, that's the real question. I wanted to mention that in the description, but got carried away. 🙈
It might actually be better to keep the original text and only add a note that refers to the updated information (e.g. https://crystal-lang.org/api/Fiber/ExecutionContext.html#the-default-execution-context).