tutorial.md: tiny correction for --crate-type=lib

Compiling a crate with `rustc --crate-type=lib` produces an rlib file, not a shared object. The latter is only produced for `--crate-type=dylib`.
auto merge of #14970 : nkoep/rust/patch-1, r=huonw

Compiling a crate with `rustc --crate-type=lib` produces an rlib file, not a shared object. The latter is only produced for `--crate-type=dylib`.