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.
Ah I see, woops, didn't realize.  When I have a chance later, I'll remove those and see if there's any other symols I might be able to add.  From what I could tell it seemed fairly robust already.