Link of `ndk-examples` is invalid in `README.md`

[wuwbobo2021]

This address is currently invalid: https://github.com/rust-mobile/cargo-apk/tree/main/examples/examples

Please change it to: https://github.com/rust-mobile/cargo-apk/tree/main/ndk-examples

PS: I don't know where ndk-context crate is located in the repositories.