bugfix for value length in button elements

[avanderm]

Summary

Bugfix for max value length in button elements. Documentation indicates 2000, the code however permits no more than 75 characters.

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #514 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   69.11%   69.23%   +0.11%     
==========================================
  Files          15       15              
  Lines        1690     1690              
  Branches       93       93              
==========================================
+ Hits         1168     1170       +2     
+ Misses        499      498       -1     
+ Partials       23       22       -1

Impacted Files	Coverage Δ
slack/web/classes/elements.py	85.54% <100%> (ø)	⬆️
slack/rtm/client.py	84.23% <0%> (+1.08%)	⬆️

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[RodneyU215]

Thanks for this PR @avanderm! Great catch.