Typos in README: 'defi primatives' and 'used pricing formula is documented'

[illia-fz]

Hi team,

I noticed a couple of minor typos in the README:

• The introduction says: "This repo is dedicated to making minimal repos of existing defi primatives." It should be "DeFi primitives" (capitalization and correct spelling).

• In the list of completed contracts, the description for Exchange.sol, Factory.sol, and Token.sol states "The used pricing formula is documented here". To improve readability, please remove the word "used" so that it reads "The pricing formula is documented here".

I'm happy to open a PR with these fixes if that would be helpful.

Thanks for the great work!