Update Recursion example to demonstrate recursive closures

Fixes #389

examples/recursion/recursion.go

@@ -17,4 +17,21 @@ func fact(n int) int {
 
 func main() {
 	fmt.Println(fact(7))
+
+	// Closures can also be recursive, but this requires the
+	// closure to be declared with a typed `var` explicitly
+	// before it's defined.
+	var fib func(n int) int
+
+	fib = func(n int) int {
+		if n < 2 {
+			return n
+		}
+		return fib(n-1) + fib(n-2)
+
+		// Since `fib` was previously declared in `main`, Go
+		// knows which function to call with `fib` here.
+	}
+
+	fmt.Println(fib(7))
 }