readme: use deadcode -test

Co-Authored-By: Warp <agent@warp.dev>

Author: croaky

README.md

@@ -292,7 +292,7 @@ cat example.md | mdembed | mods
 goimports -local "$(go list -m)" -w .
 go vet ./...
 go test ./...
-deadcode .
+deadcode -test ./...
 
 # commit
 git add -A