[chore] update readme.md

Author: Gr1N

README.md

@@ -22,17 +22,16 @@ from pynats import NATSClient
 with NATSClient() as client:
     # Connect
     client.connect()
-    
+
     # Subscribe
     def callback(msg):
-        message = '%s' % msg
-        print('Received a message with subject ' + msg.subject + ': ' + message)
+        print(f"Received a message with subject {msg.subject}: {msg}")
 
     client.subscribe(subject="test-subject", callback=callback)
-   
+
     # Publish a message
     client.publish(subject="test-subject", payload=b"test-payload")
-    
+
     # wait for 1 message
     client.wait(count=1)
 ```