Fix async_wait_message docstring

[nklokk]

Typo fix in the word "message".

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Please check the type of change your PR introduces:

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [x] Documentation content changes
• [ ] Other (please describe):

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