docs(halfslice): fix subject

- upgrade intructions
- add missing import 'piscine'
- fix console and its output
- fix white-spaces

subjects/halfslice/README.md

@@ -2,7 +2,7 @@
 
 ### Instructions
 
-Write a function that receives a slice with integers and returns a new one with the first half of the values. If the length is odd, round it up.
+Write a function `HalfSlice()` that receives a slice of `int` and returns another slice of `int` with the first half of the values. If the length is odd, round it up.
 
 ### Expected function
 
@@ -14,25 +14,26 @@ func HalfSlice(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(HalfSlice([]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}))
-	fmt.Println(HalfSlice([]int{1, 2, 3}))
+	fmt.Println(piscine.HalfSlice([]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}))
+	fmt.Println(piscine.HalfSlice([]int{1, 2, 3}))
 }
 ```
 
-And its output :
+And its output:
 
 ```console
-$ go run .
-[1,2,3,4,5]
-[1,2]
+$ go run . | cat -e
+[1 2 3 4 5]
+[1 2]
 ```