`vec!` is used early in the slides

[amanjeev]

Where

In the slide titled: Shorthand: while let conditionals

What

Code uses vec!

fn main() {
    let mut numbers = vec![1, 2, 3];  // <----- not good here because used early and potentially confusing
    while let Some(num) = numbers.pop() {
        println!("popped number {}", num);
    }
}

Fix

1. use the usual non-macro way to create Vec
2. perhaps move this slide later?

   ### Tasks
   - [ ] use the usual non-macro way to create Vec
   - [ ] use the usual non-macro way to create Vec