fix tests

Author: petitphp

features/cache-pluck.feature

@@ -35,6 +35,6 @@ Feature: Pluck command available for the object cache
     When I try `wp cache pluck unknown_key test`
     Then STDERR should be:
       """
-      Warning: No object found for the key "unknown_key" in group "default"
+      Warning: No object found for the key 'unknown_key' in group 'default'
       """