[x/tour] rename k to i in range example

X-Tour-Commit: a7fa439795f7abe5a7b14f3dfdcfc0029cfdb226

tour/static/index.html

@@ -1021,8 +1021,8 @@ import "fmt"
 var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
 
 func main() {
-	for k, v := range pow {
-	    fmt.Printf("2**%d = %d\n", k, v)
+	for i, v := range pow {
+	    fmt.Printf("2**%d = %d\n", i, v)
 	}
 }
 </div>