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 went off of what is supported in LaTeX. \R is double struck R which is the set of all real numbers, \Z is double struck Z which is the set of integers; Q is rational numbers, and N is natural numbers. They are used fairly heavily in proofs.