a iteration can be saved for "func (r *Rand) Perm(n int) []int"

In `src/math/rand/rand.go`, there is the following code:

```
func (r *Rand) Perm(n int) []int {
    m := make([]int, n)
    for i := 0; i < n; i++ {
        j := r.Intn(i + 1)
        m[i] = m[j]
        m[j] = i
    }
    return m
}
```

`i` can start from `1` instead of `0` since the iteration for `i=0` always swap `m[0]` with `m[0]`. A fix is to change `i := 0` to `i := 1`.
