Add macOS path to config.py's docstring (#501)

Author: 0xItx

examples/ptpython_config/config.py

@@ -3,6 +3,7 @@
 
 Copy this file to $XDG_CONFIG_HOME/ptpython/config.py
 On Linux, this is: ~/.config/ptpython/config.py
+On macOS, this is: ~/Library/Application Support/ptpython/config.py
 """
 from prompt_toolkit.filters import ViInsertMode
 from prompt_toolkit.key_binding.key_processor import KeyPress