[Swift] Trailing closures

[julien-c]

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html#//apple_ref/doc/uid/TP40014097-CH11-ID102

I love the Swift syntax for parameters in trailing closures and thought it'd be helpful to list it too:

let strings = numbers.map { n in
    return n**2
}