docs(swapfirst): fix subject

- fix exercise title
- upgrade instructions
- add missing import 'piscine'
- fix white-spaces and indentation

subjects/swapfirst/README.md

@@ -1,8 +1,9 @@
-## swap-first
+## swapfirst
 
 ### Instructions
 
-- Write a function that takes a slice in parameter and swaps the two first element then return the new value 
+Write a function that takes a slice of integers as an argument and returns another slice of integers with the two first elements swapped.
+
 - If the slice contains less than two elements return the same slice.
 
 ### Expected function
@@ -15,27 +16,28 @@ func SwapFirst(slice []int) []int {
 
 ### Usage
 
-Here is a possible program to test your function :
+Here is a possible program to test your function:
 
 ```go
 package main
 
 import (
 	"fmt"
+	"piscine"
 )
 
 func main() {
-    fmt.Println(SwapFirst([]int{1,2,3,4}))
-    fmt.Println(SwapFirst([]int{3,4}))
-    fmt.Println(SwapFirst([]int{1}))
+	fmt.Println(piscine.SwapFirst([]int{1, 2, 3, 4}))
+	fmt.Println(piscine.SwapFirst([]int{3, 4, 6}))
+	fmt.Println(piscine.SwapFirst([]int{1}))
 }
 ```
 
-And its output :
+And its output:
 
 ```console
 $ go run .
-[2,1,3,4]
-[4,3]
+[2 1 3 4]
+[4 3 6]
 [1]
 ```