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.
I don't think the mathalpha group matters, it's probably the same as an ord. The problem is that the notation of `\R` isn't standard, people often use `\mathbb{R}` (as suggested in the document you linked to).