fix a typo in Slack error message

[bobchao]

Viber -> Slack.

[codecov-io]

Codecov Report

Merging #671 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #671   +/-   ##
=====================================
  Coverage      81%    81%           
=====================================
  Files         101    101           
  Lines        4312   4312           
  Branches     1080   1069   -11     
=====================================
  Hits         3493   3493           
- Misses        770    775    +5     
+ Partials       49     44    -5

Impacted Files	Coverage Δ
packages/bottender/src/slack/SlackConnector.ts	93.45% <ø> (ø)	⬆️
packages/bottender/src/getClient.ts	52.94% <0%> (ø)	⬆️
packages/create-bottender-app/src/shared/log.ts	0% <0%> (ø)	⬆️

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[chentsulin]

@bobchao Thanks!