docs(concatalternate): instructions upgrade

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

subjects/concatalternate/README.md

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+## concatalternate
+
+### Instructions
+
+Write a function `ConcatAlternate()` that receives two slices of an `int` as arguments and returns a new slice with the result of the alternated values of each slice.
+- The input slices can be of different lengths.
+- The new slice should start with an element of the largest slice.
+- If the slices are of equal length, the new slice should return the elements of the first slice first and then the elements of the second slice.
+
+### Expected function
+
+```go
+func ConcatAlternate(slice1,slice2 []int) []int {
+
+}
+```
+
+### Usage
+
+Here is a possible program to test your function:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	fmt.Println(piscine.ConcatAlternate([]int{1, 2, 3}, []int{4, 5, 6}))
+	fmt.Println(piscine.ConcatAlternate([]int{2, 4, 6, 8, 10}, []int{1, 3, 5, 7, 9, 11}))
+	fmt.Println(piscine.ConcatAlternate([]int{1, 2, 3}, []int{4, 5, 6, 7, 8, 9}))
+	fmt.Println(piscine.ConcatAlternate([]int{1, 2, 3}, []int{}))
+}
+```
+
+And its output:
+
+```console
+$ go run .
+[1 4 2 5 3 6]
+[1 2 3 4 5 6 7 8 9 10 11]
+[4 1 5 2 6 3 7 8 9]
+[1 2 3]
+```