fix(readme): link to main.ts broken

[daniel-lanciana]

Should be https://github.com/lrettig/wrc20-challenge/blob/master/assembly/src/main.ts (missing /src/)

ps. didn't want to fork project and create a PR just for a broken link

[lrettig]

Thanks. This is going to be replaced with a much more comprehensive repo soon so I'll just leave it for now.