Remove "bin" section from Cargo.toml
auto merge of #16215 : cakebaker/rust/remove_bin_section_from_toml, r=alexcrichton

According to the [source code](https://github.com/rust-lang/cargo/blob/master/src/cargo/ops/cargo_new.rs#L31) <code>cargo new</code> doesn't generate a <code>[[bin]]</code> section in the <code>Cargo.toml</code>, hence removing it from the example.