Make external-types.md compile with gleam 0.3

[chiroptical]

I looked at the order of push and noticed it was wrong. Then, I wanted to play with the example and it didn't compile. This should compile and work now. Unsure if any other changes need to be made.

chiroptical@Barrys-MacBook-Air ~/p/g/test (main)> cat src/test.gleam
import gleam/io

pub type Queue(a)

@external(erlang, "queue", "new")
pub fn new() -> Queue(a)

@external(erlang, "queue", "len")
pub fn length(queue: Queue(a)) -> Int

@external(erlang, "queue", "in")
pub fn push(elem: a, queue: Queue(a)) -> Queue(a)


pub fn main() {
  let queue = new()
  io.debug(length(queue))
  let new_queue = push(0, queue)
  io.debug(length(new_queue))
}
chiroptical@Barrys-MacBook-Air ~/p/g/test (main)> gleam --version
gleam 0.30.0
chiroptical@Barrys-MacBook-Air ~/p/g/test (main)> gleam run
  Compiling gleam_stdlib
  Compiling gleeunit
  Compiling test
   Compiled in 0.01s
    Running test.main
0
1

[chiroptical]

I ran make with mdbook, but my guess is that the version I got is different and made a lot of other changes.

I only accepted the obvious changes.

[chiroptical]

Happy to change the name new here.

[chiroptical]

Closes #256 (I think)

[lpil]

Amazing, thank you!