Link out to wiki page for the concept of a "generator"

The term "generator" is used in the documentation, but is never defined.

_overviews/scala3-book/control-structures.md

@@ -236,7 +236,7 @@ for i <- ints do println(i)
 {% endtabs %}
 
 
-The code `i <- ints` is referred to as a _generator_.
+The code `i <- ints` is referred to as a [_generator_](https://en.wikipedia.org/wiki/Generator_(computer_programming)).
 
 This is what the result looks like in the Scala REPL: