Clean up awkward sentence in docs.

[bzmw]

Pretty simple, just making this sentence a bit easier to read and understand who gets the tokens.

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.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[filmaj]

Thanks for the PR! If you could sign the CLA as per the CLAassistant's comment, I will merge the PR.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @bzmw to sign the Salesforce Inc. Contributor License Agreement.