Update 07.2.md

勘误

eBook/07.2.md

@@ -121,7 +121,7 @@ func sum(a []int) int {
 	return s
 }
 
-func main {
+func main() {
 	var arr = [5]int{0, 1, 2, 3, 4}
 	sum(arr[:])
 }