update ConfigObj.write() documentation

[udance4ever]

per #258 (comment)

documentation is invalid: file_object key no longer exists

https://configobj.readthedocs.io/en/latest/configobj.html#write

Mentioning (and/or providing an example) that file objects must be opened in binary mode (for write) and that this differs in usage from the built-in ConfigParser object:

https://docs.python.org/3/library/configparser.html#quick-start

Brownie points for documenting the rationale behind this choice :)