fix #196

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() {