Update nruneprog.en.md

subjects/nruneprog.en.md

@@ -15,16 +15,7 @@ This means that:
 
 Write a function that returns the nth `rune` of a `string`.
 
-### Expected function
-
-```go
-func NRune(s string, n int) rune {
-
-}
-```
-### Instructions
-
-Write a function that returns the nth `rune` of a `string`.
+- In case of impossibilities, the function returns `0`.
 
 ### Expected function
 
@@ -49,6 +40,8 @@ import (
 func main() {
 	z01.PrintRune(piscine.NRune("Hello!", 3))
 	z01.PrintRune(piscine.NRune("Salut!", 2))
+	z01.PrintRune(piscine.NRune("Bye!", -1))
+	z01.PrintRune(piscine.NRune("Bye!", 5))
 	z01.PrintRune(piscine.NRune("Ola!", 4))
 	z01.PrintRune('\n')
 }