docs(firstword): require a function instead of a program

subjects/firstword/README.md

@@ -2,22 +2,44 @@
 
 ### Instructions
 
-Write a program that takes an argument and displays its first word, followed by a newline (`'\n'`).
+Write a function that takes a string and return a string containing its first word, followed by a newline (`'\n'`).
 
 - A word is a sequence of characters delimited by spaces or by the start/end of the argument.
 
-- The output will be followed by a newline (`'\n'`).
+### Expected Function
 
-- If the number of arguments is not 1, or if there are no words, the program displays nothing.
+```go
+func FirstWord(s string) string {
+    // ...
+}
+```
 
 ### Usage
 
+Here is a possible way to test your function:
+
+```go
+package main
+
+import (
+    "fmt"
+
+    "piscine"
+)
+
+func main() {
+    fmt.Print(piscine.FirstWord("hello there"))
+    fmt.Print(piscine.FirstWord(""))
+    fmt.Print(piscine.FirstWord("hello   .........  bye"))
+}
+```
+
+And its output:
+
 ```console
-$ go run . "hello there"
+$ go run .
 hello
-$ go run . "hello   .........  bye"
+
 hello
-$ go run .
-$ go run . "hello" "there"
 $
 ```