Fix syntax highlighting of code fence
Fix syntax highlighting of code fence

The documentation for RESULT_EXPECT_USED includes this code:

    let res: Result<usize, ()> = Ok(1);
    res?;
    # Ok::<(), ()>(())

Because the code fence didn't start with `rust`, the code wasn't highlighted and the line starting with `#` was displayed on the website. This is now fixed.
graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust