Fix source url

Author: ErikSchierboom

exercises/split-second-stopwatch/metadata.toml

@@ -1,4 +1,4 @@
 title = "Split-Second Stopwatch"
 blurb = "Keep track of time through a digital stopwatch."
 source = "Erik Schierboom"
-source_url = "https://github.com/exercism/problem-specifications/pull/TODO"
+source_url = "https://github.com/exercism/problem-specifications/pull/2547"