Sphinx warning in generated clients with mixins

[parthea]

In PR googleapis/python-network-security#115, there was a sphinx warning. This warning appears in all generated clients with mixins. See the stack trace below and build log here:
https://github.com/googleapis/python-network-security/runs/7482455566?check_suite_focus=true

Traceback (most recent call last):
  File "/home/runner/work/python-network-security/python-network-security/.nox/docs/lib/python3.8/site-packages/sphinx/cmd/build.py", line 280, in build_main
    app.build(args.force_all, filenames)
  File "/home/runner/work/python-network-security/python-network-security/.nox/docs/lib/python3.8/site-packages/sphinx/application.py", line 350, in build
    self.builder.build_update()
  File "/home/runner/work/python-network-security/python-network-security/.nox/docs/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 292, in build_update
    self.build(to_build,
  File "/home/runner/work/python-network-security/python-network-security/.nox/docs/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 306, in build
    updated_docnames = set(self.read())
  File "/usr/lib/python3.8/contextlib.py", line 120, in __exit__
    next(self.gen)
  File "/home/runner/work/python-network-security/python-network-security/.nox/docs/lib/python3.8/site-packages/sphinx/util/logging.py", line 213, in pending_warnings
    memhandler.flushTo(logger)
  File "/home/runner/work/python-network-security/python-network-security/.nox/docs/lib/python3.8/site-packages/sphinx/util/logging.py", line 178, in flushTo
    logger.handle(record)
  File "/usr/lib/python3.8/logging/__init__.py", line 1599, in handle
    self.callHandlers(record)
  File "/usr/lib/python3.8/logging/__init__.py", line 1661, in callHandlers
    hdlr.handle(record)
  File "/usr/lib/python3.8/logging/__init__.py", line 950, in handle
    rv = self.filter(record)
  File "/usr/lib/python3.8/logging/__init__.py", line 811, in filter
    result = f.filter(record)
  File "/home/runner/work/python-network-security/python-network-security/.nox/docs/lib/python3.8/site-packages/sphinx/util/logging.py", line [42](https://github.com/googleapis/python-network-security/runs/7482455566?check_suite_focus=true#step:5:43)3, in filter
    raise exc
sphinx.errors.SphinxWarning: /home/runner/work/python-network-security/python-network-security/google/cloud/network_security_v1beta1/services/network_security/async_client.py:docstring of google.cloud.network_security_v1beta1.services.network_security.async_client.NetworkSecurityAsyncClient.get_iam_policy:33:Unexpected indentation.
Warning, treated as error:
/home/runner/work/python-network-security/python-network-security/google/cloud/network_security_v1beta1/services/network_security/async_client.py:docstring of google.cloud.network_security_v1beta1.services.network_security.async_client.NetworkSecurityAsyncClient.get_iam_policy:33:Unexpected indentation.
nox > Command sphinx-build -W -T -N -b html -d docs/_build/doctrees/ docs/ docs/_build/html/ failed with exit code 2
nox > Session docs failed.
Error: Process completed with exit code 1.