fix astaxie#196

Author: astaxie

ebook/02.7.md

@@ -68,11 +68,11 @@ channel通过操作符`<-`来接收和发送数据
 import "fmt"
 
 func sum(a []int, c chan int) {
-sum := 0
+total := 0
 for _, v := range a {
-sum += v
+total += v
 }
-c <- sum // send sum to c
+c <- total // send total to c
 }
 
 func main() {