fx formatting of the error log for flag key (#381)

Author: Mat001

optimizely/optimizely.py

@@ -1009,7 +1009,7 @@ def _decide(self, user_context, key, decide_options=None):
 
         feature_flag = config.get_feature_from_key(key)
         if feature_flag is None:
-            self.logger.error("No feature flag was found for key '#{key}'.")
+            self.logger.error(f"No feature flag was found for key '{key}'.")
             reasons.append(OptimizelyDecisionMessage.FLAG_KEY_INVALID.format(key))
             return OptimizelyDecision(flag_key=key, user_context=user_context, reasons=reasons)