Update on_success docstring

[jnafzig]

Describe the bug

The on_success method is being passed the actual successful events:

snowplow-python-tracker/snowplow_tracker/emitters.py

Line 330 in cb7e434

self.on_success(success_events)

However, the docs here

snowplow-python-tracker/snowplow_tracker/emitters.py

Line 86 in cb7e434

Gets passed the number of events flushed.

say that this function will be passed the "number of events flushed"

The docs here are similarly incorrect:
https://docs.snowplow.io/docs/collecting-data/collecting-from-own-applications/python-tracker/emitters/#on_success

To Reproduce
The example in the online docs under on_failure reproduce the problem: https://docs.snowplow.io/docs/collecting-data/collecting-from-own-applications/python-tracker/emitters/#on_failure

Expected behavior
The docs should match the behavior.