docs(concatslice): fix subject

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

subjects/concatslice/README.md

@@ -1,8 +1,8 @@
-## Concat-Slice
+## concatslice
 
 ### Instructions
 
-- Write a function that takes two slices of integers and returns the concatenation of the two slices. 
+Write a function `ConcatSlice()` that takes two slices of integers as arguments and returns the concatenation of the two slices.
 
 ### Expected function
 
@@ -21,12 +21,13 @@ package main
 
 import (
 	"fmt"
+	"piscine"
 )
 
 func main() {
-	fmt.Println(ConcatSlice([]int{1,2,3},[]int{4,5,6}))
-    fmt.Println(ConcatSlice([]int{},[]int{4,5,6,7,8,9}))
-    fmt.Println(ConcatSlice([]int{1,2,3},[]int{}))
+	fmt.Println(piscine.ConcatSlice([]int{1, 2, 3}, []int{4, 5, 6}))
+	fmt.Println(piscine.ConcatSlice([]int{}, []int{4, 5, 6, 7, 8, 9}))
+	fmt.Println(piscine.ConcatSlice([]int{1, 2, 3}, []int{}))
 }
 ```