CI: Update dvc's argument --show-md to --md for dvc 3.x (#2583)

Author: seisman

.github/workflows/dvc-diff.yml

@@ -65,7 +65,7 @@ jobs:
 
         # Pull image data from cloud storage
         dvc pull --remote upstream
-        dvc diff --show-md main HEAD >> report.md
+        dvc diff --md main HEAD >> report.md
 
         # Get just the filename of the added and modified image from the report
         awk 'NF==5 && NR>=7 && $2=="added" {print $4}' report.md > added_files.txt