Update introduction.md

Co-authored-by: András B Nagy <[email protected]>

Author: ErikSchierboom

exercises/split-second-stopwatch/introduction.md

@@ -2,5 +2,5 @@
 
 You've always run for the thrill of it — no schedules, no timers, just the sound of your feet on the pavement.
 But now that you've joined a competitive running crew, things are getting serious.
-Training sessions are timed to the second, and every split counts.
+Training sessions are timed to the second, and every split second counts.
 To keep pace, you've picked up the _Split-Second Stopwatch_ — a sleek, high-tech gadget that's about to become your new best friend.