Add a missing semicolon to the hello world program in the docs.

Add a missing semicolon to the hello world program in the docs.

Both the `index.md` and `guide.md` files show the hello world program source
(generated by `cargo new`) without a semicolon (`;`) at the end of the
`println!` line. The file generated by `cargo new` does and should have a
semicolon. This commit brings the docs in sync with the generated code.