Update beta releasing steps for maintainers

[srajiang]

Summary

Mirrors the changes to node-slack-sdk beta release instructions to use npm publish --tag and removes the need to invoke npm dist-tag.

Requirements (place an x in each [ ])

• 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

Merging #1348 (d3c682f) into main (f3536d6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1348   +/-   ##
=======================================
  Coverage   72.76%   72.76%           
=======================================
  Files          17       17           
  Lines        1465     1465           
  Branches      436      436           
=======================================
  Hits         1066     1066           
  Misses        310      310           
  Partials       89       89           

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[seratch]

Thanks for fixing this!