update book code

eBook/examples/chapter_6/function_closure.go

@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+func main() {
+	var f = Adder()
+	fmt.Print(f(1)," - ")
+	fmt.Print(f(20)," - ")
+	fmt.Print(f(300))
+}
+
+func Adder() func(int) int {
+	var x int
+	return func(delta int) int {
+		x += delta
+		return x
+	}
+}