docs(rectperimeter): fix subject

- added missing information and usage
- fixed grammar and indentation

subjects/rectperimeter/README.md

@@ -1,27 +1,29 @@
-# rectperimeter
+## rectperimeter
 
 ### Instructions
 
-write a function that accepts two integer arguments, representing the two sides length of a rectangle and returning the perimeter of the rectangle.
+Write a function that takes two `int`'s as arguments, representing the length of width and height of a rectangle and returning the perimeter of the rectangle.
 
-your function should return -1 if one of the arguments is negative.
+- If one of the arguments is negative it should return `-1`.
 
 ### Expected function
 
 ```go
-func RectPerimeter(l, w int) int {
+func RectPerimeter(w, h int) int {
 
 }
 ```
 
 ### Usage
 
+Here is a possible program to test your function:
 
 ```go
 package main
 
 import (
 	"fmt"
+
 	"piscine"
 )
 
@@ -30,3 +32,12 @@ func main() {
 	fmt.Println(piscine.RectPerimeter(434343, 898989))
 }
 ```
+
+And its output:
+
+```console
+$ go run .
+24$
+2666664$
+$
+```