Add "initial diffing" commit URL

.github/workflows/periodic.yml

@@ -3,7 +3,7 @@
 
 # For future reference (in case we're someday able to revert this):
 #   - https://github.com/docker-library/official-images/commit/a40bfb3617fb765ccabf9794e55f7cece6281696 (initial labelling)
-#   - TODO (initial diffing)
+#   - https://github.com/docker-library/official-images/commit/088d51bfde4ea9019dd9c7457cb7b5510bf1e04e (initial diffing)
 
 name: Periodic Actions