Simplify the launcher

Having `if __name__ == "__main__"` in `__main__.py` is redundant,
because the `greetings` module itself is runnable due to the mere
existence of the `__main__.py` file.

Importing just 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 +90,11 @@ so initialize the command-line interface here:
 
 .. code-block:: python
 
-	if __name__ == "__main__":
-	    from greetings.cli import app
-	    app()
+    import sys
+
+    from greetings.cli import main
+
+    sys.exit(main())
 
 .. note::