Fix #1207 by correcting the typing introduced by #1181

[seratch]

This pull request resolves #1207, which is a bug caused by #1181

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

• [ ] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [x] slack-api-client (Slack API Clients)
• [x] slack-api-model (Slack API Data Models)
• [ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [ ] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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

[codecov[bot]]

Codecov Report

Merging #1208 (2bc884f) into main (a1720ec) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1208   +/-   ##
=========================================
  Coverage     74.87%   74.87%           
  Complexity     4074     4074           
=========================================
  Files           436      436           
  Lines         12835    12835           
  Branches       1301     1301           
=========================================
  Hits           9610     9610           
  Misses         2464     2464           
  Partials        761      761           

Files Changed	Coverage Δ
.../src/main/java/com/slack/api/model/Attachment.java	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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