bisect-kit: fix lint warnings

BUG=None
TEST=pylint

Change-Id: I2add8a04b49d7cc432f138c6d0b76eb880485b98
diff --git a/bisect_kit/cros_util.py b/bisect_kit/cros_util.py
index dfcd7f0..0eed6e6 100644
--- a/bisect_kit/cros_util.py
+++ b/bisect_kit/cros_util.py
@@ -1906,7 +1906,7 @@
         reference=self.config['chromeos_mirror'],
         # b/150753074: moblab is in non-default group and causes mark_as_stable
         # fail
-        groups="default,moblab,platform-linux",
+        groups='default,moblab,platform-linux',
     )
 
     # Note, don't sync with current_branch=True for chromeos. One of its