docs(types): fix markdown block type annotation

[zimeg]

Summary

This PR fixes the markdown block type annotation to match documentation. Following a pasting oops of #2296.

Notes

The type of block. For a markdown block, type is always markdown.

🔗 https://docs.slack.dev/reference/block-kit/blocks/markdown-block#fields

Requirements

• 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.71%. Comparing base (e694d58) to head (39ce29b).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2298   +/-   ##
=======================================
  Coverage   92.71%   92.71%           
=======================================
  Files          38       38           
  Lines       10607    10607           
  Branches      683      683           
=======================================
  Hits         9834     9834           
  Misses        761      761           
  Partials       12       12           

Flag	Coverage Δ
cli-hooks	95.23% <ø> (ø)
cli-test	94.74% <ø> (ø)
oauth	77.39% <ø> (ø)
socket-mode	61.87% <ø> (ø)
web-api	97.97% <ø> (ø)
webhook	96.66% <ø> (ø)

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