docs: correct required app permissions in setup description

[CCFenner]

The linked guide to setup GH apps for authentication is missing the checks permission. Without that the plugin is not able to report back to GH. This changes explicitly states this in the setup description.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

Merging #321 (5b85029) into master (a51e30c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #321   +/-   ##
=========================================
  Coverage     80.79%   80.79%           
  Complexity      170      170           
=========================================
  Files            16       16           
  Lines           526      526           
  Branches         49       49           
=========================================
  Hits            425      425           
  Misses           79       79           
  Partials         22       22           

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[jglick]

See also jenkinsci/github-branch-source-plugin#746.