RecreateSKPs: Fix another .join() issue

BUG=skia:5843
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2429513002

Review-Url: https://codereview.chromium.org/2429513002
diff --git a/tools/skp/webpages_playback.py b/tools/skp/webpages_playback.py
index 2e2b6a4..e8c049c 100644
--- a/tools/skp/webpages_playback.py
+++ b/tools/skp/webpages_playback.py
@@ -493,7 +493,7 @@
 
   def upload_dir_contents(self, source_dir, dest_dir):
     subprocess.check_call([
-        'gsutil', 'cp', '-r', source_dir, '/'.join(self._url, dest_dir)])
+        'gsutil', 'cp', '-r', source_dir, '/'.join((self._url, dest_dir))])
 
 
 class LocalFileSystemDataStore(DataStore):