Fix ResetLine by using string formatting

[tfogo]

Hi! It looks like ResetLine wasn't working as expected. See this example code:

package main

import (
  "github.com/buger/goterm"
)

func main() {
  goterm.Println("First Line")
  goterm.Flush()
  goterm.MoveCursorUp(2)
  goterm.Println(goterm.ResetLine("Second Line"))
  goterm.Flush()
}

The expected output is: Second Line but instead the output is %!(EXTRA string=Second Line) since the Sprintf looks incorrect in ResetLine.