docs(thirdtimeisthecharm): grammar fix

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

subjects/thirdtimeisacharm/README.md

@@ -0,0 +1,47 @@
+## thirdtimeisacharm
+
+### Instructions
+
+Write a function `ThirdTimeIsACharm()` that takes a `string` as an argument and returns another `string` with every third character.
+
+- Return 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:
+
+```go
+package main
+
+import (
+	"fmt"
+	"piscine"
+)
+
+func main() {
+	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:
+
+```console
+$ go run . | cat -e
+369$
+$
+b e$
+$
+```