util: Fix number format for `pad`

`pad` is now using `toString(10)`, actually we don't need to do this. As for https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toString, `toString(N)` is a radix converter, which isn't proper here for time conversion.

PR-URL: https://github.com/nodejs/node/pull/21906


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Ruby <rubys@intertwingly.net>