Fix inline docs for InstallationEvent type

[andygrunwald]

What type of PR is this?

documentation

What this PR does / why we need it:

The inline documentation for the InstallationEvent went out of date.
This PR fixes the link + possible values.

Furthermore, does the REST API send a requester key as well.
In my current use-case, this was null, hence I am not aware of the type (is not documented).
As a trade-off for now, I added a TODO. Once the type is known, this can be extended

Special notes for your reviewer:

This applies the same fix as in #1780

Does this PR introduce a user-facing change?:

None

Additional documentation e.g., usage docs, etc.:

• Webhook events and payloads: installation

[codecov]

Codecov Report

Merging #1808 (5944538) into master (5c87615) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1808   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files         100      100           
  Lines        6521     6521           
=======================================
  Hits         6364     6364           
  Misses         85       85           
  Partials       72       72           

Impacted Files	Coverage Δ
github/event_types.go	100.00% <ø> (ø)

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[gmlewis]

Thank you, @andygrunwald !
LGTM.
Merging.