wrong link in README.md

[DGCK81LNN]

https://github.com/wenyan-lang/book#read-online

## [Read Online](https:/book.wy-lang.org)

this link currently goes to https://github.com/book.wy-lang.org because of missing slash, should be:

## [Read Online](https://book.wy-lang.org)

[Lingdong Huang]

Thanks a lot for pointing this out!

Fixed: 03606e5
and again: 01c1a48