Add `move` keyword for closures to the Reference

[cbreeden]

Currently the rust reference doesn't mention the move keyword used in snippets like this:

let mut add_num = move |x: i32| num += x;

[chuck-park]

https://doc.rust-lang.org/grammar.html#keywords includes move in the list of keywords.

[steveklabnik]

This has now been fixed, thank you!

[paulie4]

NOTE: the new location for the added move documentation is here:
https://doc.rust-lang.org/reference/expressions/closure-expr.html