chore(storage): fix lint error in _signing.py module (googleapis#140)

Fixes googleapis#139 

This PR fixes a lint error that recently started occurring on master out of the blue.
### How to verify

Run the lint check on master:

    $ nox -f noxfile.py -s lint

google/cloud/storage/_signing.py

@@ -153,9 +153,7 @@ def get_expiration_seconds_v4(expiration):
 
     if seconds > SEVEN_DAYS:
         raise ValueError(
-            "Max allowed expiration interval is seven days (%d seconds)".format(
-                SEVEN_DAYS
-            )
+            "Max allowed expiration interval is seven days {}".format(SEVEN_DAYS)
         )
 
     return seconds