Minor: Correcting documentation of https-proxy-agent usage for slack-sdk

[rajashekar]

Summary

Currently in documentation https-proxy-agent module is imported like below
const { HttpsProxyAgent } = require('https-proxy-agent');
Which will cause HttpsProxyAgent come as undefined
which will cause below error on new HttpsProxyAgent(process.env.http_proxy || 'http://proxyserver:8080');

TypeError: HttpsProxyAgent is not a constructor
at Object. (/home/rchint1/workspace/Release-Police-mk3/index.js:15:15)
at Module._compile (module.js:643:30)
at Object.Module._extensions..js (module.js:654:10)
at Module.load (module.js:556:32)
at tryModuleLoad (module.js:499:12)
at Function.Module._load (module.js:491:3)
at Function.Module.runMain (module.js:684:10)
at startup (bootstrap_node.js:187:16)
at bootstrap_node.js:608:3

Refer https proxy agent api doc - https://www.npmjs.com/package/https-proxy-agent
below is the right way to import
const HttpsProxyAgent = require('https-proxy-agent');

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #811 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #811   +/-   ##
=======================================
  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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[codecov]

Codecov Report

Merging #811 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #811   +/-   ##
=======================================
  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)	⬆️

---

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 8266961...f245ba4. Read the comment docs.

[rajashekar]

Note: After raising this PR, I found out that there was another PR - #787 to fix this, but was not merged yet. So keeping this PR open to see if this can be merged once all checks are done.

[clavin]

👍 Nice catch! Thank you for bettering our documentation (and fixing the examples)!

(Failing integration tests related to #834)