added a possibly missing exclamation mark

In the previous article, in a similar example, he was

1-js/02-first-steps/17-arrow-functions-basics/article.md

@@ -64,7 +64,7 @@ For instance, to dynamically create a function:
 let age = prompt("What is your age?", 18);
 
 let welcome = (age < 18) ?
-  () => alert('Hello') :
+  () => alert('Hello!') :
   () => alert("Greetings!");
 
 welcome();