update book code

eBook/exercises/chapter_14/gosum.go

@@ -0,0 +1,16 @@
+// gosum.go
+package main
+
+import (
+	"fmt"
+)
+
+func sum(x, y int, c chan int) {
+	c <- x + y
+}
+
+func main() {
+	c := make(chan int)
+	go sum(12, 13, c)
+	fmt.Println(<-c)  // 25
+}