[py] fix sdist publish instructions (SeleniumHQ#11375)

It appears that only wheels are being published to pypi, this should
fix the publishing procedure to include sdists.

README.md

@@ -195,8 +195,8 @@ pip install bazel-bin/py/selenium-*.whl
 To publish run:
 
 ```sh
-bazel build //py:selenium-wheel
-twine upload bazel-bin/py/selenium-*.whl
+bazel build //py:selenium-wheel //py:selenium-sdist
+twine upload bazel-bin/py/selenium-*.whl bazel-bin/py/selenium-*.tar.gz
 ```
 </details>