fix crate_name attribute description

[neetsdkasu]

I think the underscore is correct, not the hyphen

#![crate_name = "foo-bar"]

fn main() {
    println!("Hello, world!");
}

$ rustc src\main.rs
error: invalid character `-` in crate name: `foo-bar`
 --> src\main.rs:1:1
  |
1 | #![crate_name = "foo-bar"]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

#![crate_name = "foo\u{002D}bar"]

fn main() {
    println!("Hello, world!");
}

$ rustc src\main.rs
error: invalid character `-` in crate name: `foo-bar`
 --> src\main.rs:1:1
  |
1 | #![crate_name = "foo\u{002D}bar"]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

#![crate_name = "foo_bar"] is no error
#![crate_name = "foo\u{005F}bar"] is no error

[ehuss]

Thank you! Not sure how I flubbed that.