fix typos in function helper text (#503)

* fix typos in autogen/agentchat/conversable_agent.py

* fix typos in autogen/code_utils.py

* fix typos in autogen/oai/completion.py

* fix typos in test/agentchat/test_assistant_agent.py

* Update test/agentchat/test_assistant_agent.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>