Simplify the launcher

Importing the `main()` function is likely more common and concise than
relying on the heavy-weight `app` object to be callable.

Author: anpol

source/guides/creating-command-line-tools.rst

@@ -90,9 +90,12 @@ so initialize the command-line interface here:
 
 .. code-block:: python
 
-	if __name__ == "__main__":
-	    from greetings.cli import app
-	    app()
+        import sys
+
+        if __name__ == "__main__":
+            from greetings.cli import main
+
+            sys.exit(main())
 
 .. note::