Slack: Remove parameter token in SlackAPIPostOperator's docstring

[brienzb]

Since Slack provider 8.0 version and later, parameter token has been deleted.
Related commit: ed6a4fd

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^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[hussein-awala]

Congrats on your first commit 🎉