Fix string concat in gs_utils

TBR=jcgregorio
BUG=skia:3723

Review URL: https://codereview.chromium.org/1064763003

py/utils/gs_utils.py

diff --git a/py/utils/gs_utils.py b/py/utils/gs_utils.py
index d7c01c7..c2e6de6 100644
--- a/py/utils/gs_utils.py
+++ b/py/utils/gs_utils.py
@@ -413,7 +413,7 @@
             break
           except Exception as error:
             if retry < retries - 1:
-              print '  Retrying upload, attempt #%d' % retry + 1
+              print '  Retrying upload, attempt #%d' % (retry + 1)
               time.sleep(2 ** retry)
             else:
               err[rel_path] = error