Fixed a small typo and added some symbols

Fairly insignificant typo I fixed in CONTRIBUTING.md.

I also added a number of missing symbols for number sets like real numbers, integers, natural numbers.  I also added the "parallel" symbol.
That's fine. We'd definitely like to expose an API for adding macros so people can add shorthands like this themselves in the future, but we don't want to support stuff that isn't necessary for now.