Fix #210 by correcting a type hint in async OAuth classes

[seratch]

This pull request fixes #210 - this issue does not affect anything runtime. It's just an error for type hint users.

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements (place an x in each [ ])

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

Merging #213 (23dd3aa) into main (a11f6d1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #213   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files         159      159           
  Lines        4746     4746           
=======================================
  Hits         4348     4348           
  Misses        398      398           

Impacted Files	Coverage Δ
slack_bolt/oauth/async_oauth_settings.py	100.00% <100.00%> (ø)

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