fix: link url

Author: dancergraham

chapter02_observer/readme.md

@@ -45,7 +45,7 @@ classDiagram
 
 ## Use in Python
 
-[Django signals]([url](https://docs.djangoproject.com/en/4.2/topics/signals/)https://docs.djangoproject.com/en/4.2/topics/signals/) 
+[Django signals](https://docs.djangoproject.com/en/4.2/topics/signals/)
 allow other parts of the code to sign up to receive updates, for instance when settings are changed  
 or model instances are created and / or updated.