update readme (build-exe -> build-lib)

[ghost]

This changed here: https://github.com/ziglang/zig/commit/14cdb01f35b35972b06e95a3a438f9f7910b97f8

Using build-exe will fail because it will look for a main function.

This project also needs to be updated for latest Zig syntax, but I saw there's already a PR for that.