Fixed docs issue with generateInstallUrl for OAuth

[stevengill]

Summary

Fixes issue #526

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.

[seratch]

After merging this, I will update the corresponding JA page.

[stevengill]

@seratch ready for another review

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@8bd5f78). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #527   +/-   ##
=========================================
  Coverage          ?   83.13%           
=========================================
  Files             ?        7           
  Lines             ?      593           
  Branches          ?      184           
=========================================
  Hits              ?      493           
  Misses            ?       68           
  Partials          ?       32           

Impacted Files	Coverage Δ
src/middleware/builtin.ts	82.78% <0.00%> (ø)
src/ExpressReceiver.ts	65.21% <0.00%> (ø)
src/App.ts	87.82% <0.00%> (ø)
src/middleware/process.ts	100.00% <0.00%> (ø)
src/conversation-store.ts	100.00% <0.00%> (ø)
src/errors.ts	100.00% <0.00%> (ø)
src/helpers.ts	90.90% <0.00%> (ø)

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[seratch]

LGTM - I will make another PR for the corresponding JA page.