Use the correct install command, fix: https://github.com/pycasbin/django-casbin/issues/2

Author: hsluoyz

README.md

@@ -7,7 +7,7 @@ django-casbin is an authorization middleware for [Django](https://www.djangoproj
 ## Installation
 
 ```
-pip install django-casbin
+pip install casbin
 ```
 
 ## Simple Example