Fix ChatPostMessageArguments.as_user type

[harveyr]

Summary

Fixes the type of ChatPostMessageArguments.as_user per the docs.

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

@@           Coverage Diff           @@
##           master     #475   +/-   ##
=======================================
  Coverage   80.53%   80.53%           
=======================================
  Files           5        5           
  Lines         226      226           
  Branches       35       35           
=======================================
  Hits          182      182           
  Misses         28       28           
  Partials       16       16

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce06b1e...2c13523. Read the comment docs.

[aoberoi]

good catch!!