fixes import error in @slack/webhook README

[aoberoi]

Summary

Fixes #765

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

@@           Coverage Diff           @@
##           master     #771   +/-   ##
=======================================
  Coverage   97.85%   97.85%           
=======================================
  Files          15       15           
  Lines         652      652           
  Branches       26       26           
=======================================
  Hits          638      638           
  Misses         13       13           
  Partials        1        1

Flag	Coverage Δ
#eventsapi	95.62% <ø> (ø)	⬆️
#interactivemessages	99.18% <ø> (ø)	⬆️
#webapi	98.33% <ø> (ø)	⬆️
#webhook	90.69% <ø> (ø)	⬆️

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