Flush stdout in chat_stream example in README

[srithon]

By default, without explicitly flushing stdout, Python only flushes the output buffer when it encounters newlines. This makes it look like generations are only being returned line-by-line. This misunderstanding led me to file this issue under the Golang SDK, wrongly thinking that this was an issue with the Cohere SDK as a whole rather than just being a byproduct of Python's output buffering.

[CLAassistant]

All committers have signed the CLA.