Fix log output format string

Wrong kind of brackets caused a crash when this log output was created.

Change-Id: I1be586fcd72060a5bf47d5ae7e99fac92855ff57
diff --git a/octavia_tempest_plugin/tests/validators.py b/octavia_tempest_plugin/tests/validators.py
index 2dc1d64..773fcc4 100644
--- a/octavia_tempest_plugin/tests/validators.py
+++ b/octavia_tempest_plugin/tests/validators.py
@@ -75,7 +75,7 @@
                 return
             except requests.exceptions.Timeout:
                 # Don't sleep as we have already waited the interval.
-                LOG.info('Request for () timed out. Retrying.'.format(URL))
+                LOG.info('Request for {} timed out. Retrying.'.format(URL))
             except (exceptions.InvalidHttpSuccessCode,
                     exceptions.InvalidHTTPResponseBody,
                     requests.exceptions.SSLError):