incorrect documentation for DogStatsd class - remove function flush

[gagandeepm]

Describe the bug
In DogStatsd class the function flush has been replaced by flush_buffered_metrics but the documentation is not updated
https://datadogpy.readthedocs.io/en/latest/
This is the case since version 0.50.0

To Reproduce
link to the flush function that doesn't exist anymore
https://datadogpy.readthedocs.io/en/latest/#datadog.dogstatsd.base.DogStatsd.flush

Expected behavior
Update the documentation to remove flush and replace it with flush_buffered_metrics

Environment and Versions (please complete the following information):
This is a bug from version 0.50.0

[github-actions]

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