Make the validate argument in the channels.join method optional

[DominikPalo]

Summary

The validate argument in the channels.join method is optional (see https://api.slack.com/methods/channels.join), therefore it should be marked as optional also in the associated type.

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 #530 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #530   +/-   ##
=======================================
  Coverage   79.56%   79.56%           
=======================================
  Files           6        6           
  Lines         279      279           
  Branches       43       43           
=======================================
  Hits          222      222           
  Misses         39       39           
  Partials       18       18

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[aoberoi]

thanks for finding this inconsistency!