Add required repository sections to example pom

[ClaytonHughes]

Without these, maven cannot find the parent pom. Thought I'd offer an update to the documentation to fix that problem.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
  • I have no desired changelog entry. I don't really care, just thought I'd help.
• Please describe what you did
  • I added the <repository> and <pluginRepository> sections required to get the provided pom to work.
• Link to relevant issues in GitHub or Jira
  • No issue, I just found it and fixed it.
• Link to relevant pull requests, esp. upstream and downstream changes
  • N/A
• Ensure you have provided tests - that demonstrates feature works or fixes the issue
  • Tests for a documentation change?

[codecov]

Codecov Report

Merging #864 (d5723b0) into master (4e69053) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head d5723b0 differs from pull request most recent head bcac74a. Consider uploading reports for the commit bcac74a to get more accurate results

@@             Coverage Diff              @@
##             master     #864      +/-   ##
============================================
+ Coverage     80.17%   80.20%   +0.03%     
  Complexity     1561     1561              
============================================
  Files           243      243              
  Lines          5664     5664              
  Branches        422      422              
============================================
+ Hits           4541     4543       +2     
+ Misses          966      965       -1     
+ Partials        157      156       -1     

Impacted Files	Coverage Δ	Complexity Δ
...s/plugins/analysis/core/model/ReportXmlStream.java	94.73% <0.00%> (+5.26%)	3.00% <0.00%> (ø%)

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[uhafner]

Suggested change

	<url>http://repo.jenkins-ci.org/public/</url>
	<url>https://repo.jenkins-ci.org/public/</url>