update book code

eBook/examples/chapter_10/method2.go

@@ -0,0 +1,16 @@
+package main
+
+import "fmt"
+
+type IntVector []int
+
+func (v IntVector) Sum() (s int) {
+	for _, x := range v {
+		s += x
+	}
+	return
+}
+
+func main() {
+	fmt.Println(IntVector{1, 2, 3}.Sum())  // Output: 6
+}