docs(thirdtimeisacharm): fix subject

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

subjects/thirdtimeisacharm/README.md

@@ -1,40 +1,44 @@
-# thirdtimeisacharm
+## thirdtimeisacharm
 
 ### Instructions
 
-Write a function `ThirdTimeIsACharm` that takes a string and returns another string with every third character.
+Write a function `ThirdTimeIsACharm()` that takes a `string` as an argument and returns another `string` with every third character.
 
-- Prints the output followed by newline `\n`.
-- If the string is empty, return newline `\n`.
-- If there is no third character, return newline `\n`.
+- Prints the output followed by a newline `\n`.
+- If the `string` is empty, return a newline `\n`.
+- If there is no third character, return a newline `\n`.
 
 ### Expected function
 
 ```go
 func ThirdTimeIsACharm(str string) string {
+
 }
 ```
 
 ### Usage
 
-Here is a possible program to test your function :
+Here is a possible program to test your function:
 
 ```go
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"piscine"
+)
 
 func main() {
-    fmt.Print(ThirdTimeIsACharm("123456789"))
-    fmt.Print(ThirdTimeIsACharm(""))
-    fmt.Print(ThirdTimeIsACharm("a b c d e f")) 
-    fmt.Print(ThirdTimeIsACharm("12"))
+	fmt.Print(piscine.ThirdTimeIsACharm("123456789"))
+	fmt.Print(piscine.ThirdTimeIsACharm(""))
+	fmt.Print(piscine.ThirdTimeIsACharm("a b c d e f"))
+	fmt.Print(piscine.ThirdTimeIsACharm("12"))
 }
 ```
 
-And its output :
+And its output:
 
-```go
+```console
 $ go run . | cat -e
 369$
 $