Update introduction.md

Added a ; to the do loop as it  is needed to compile

Author: joMarsch

exercises/concept/interest-is-interesting/.docs/introduction.md

@@ -39,5 +39,5 @@ do
 {
     // Execute logic if x > 10
     x = x - 2;
-} while (x > 10)
+} while (x > 10);
 ```