Fix syntax error in policy presubmit

The error string was missing the format specifier (% vs %s).

Bug: 1139306
Change-Id: I66ffb8ed02e251107879f3384ab12f34478984c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2535995Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Auto-Submit: Eric Lawrence [MSFT] <ericlaw@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#827233}

components/policy/tools/syntax_check_policy_template_json.py

@@ -1559,7 +1559,7 @@ class PolicyTemplateChecker(object):
       # TODO(crbug.com/1139306): This item check should apply to all policies
       # instead of just new ones.
       if self._NeedsItems(new_policy) and new_policy.get('items', None) == None:
-        self._Error(('Missing items field for policy %') % (new_policy_name))
+        self._Error(('Missing items field for policy %s') % (new_policy_name))
 
   def _LeadingWhitespace(self, line):
     match = LEADING_WHITESPACE.match(line)