Update Rust 1.70 in appendix/rust-versions/intro.html

[niconical]

Thank you so much for your book Rust Course , it helped me a lot. I found that Rust has been updated to version 1.70, and the appendix of this book needs to be updated to the latest version of the changelog.

Ref:

• https://blog.rust-lang.org/2023/06/01/Rust-1.70.0.html

[sunface]

got it, thanks :)