Change identifier variable from identifier to _id

[lincmba]

This is to make it consistent with other endpoints

IMPORTANT: Where possible all PRs must be linked to a Github issue

Resolves #61

Engineer Checklist

• I have written Unit tests for any new feature(s) and edge cases for
  bug fixes
• I have added documentation for any new feature(s) and configuration
  option(s) on the README.md
• I have run mvn spotless:check to check my code follows the project's
  style guide
• I have run mvn clean test jacoco:report to confirm the coverage report
  was generated at plugins/target/site/jacoco/index.html
• I ran mvn clean package right before creating this pull request.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.73%. Comparing base (d7eeb69) to head (15922cf).
Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #62      +/-   ##
============================================
- Coverage     43.81%   43.73%   -0.09%     
- Complexity      106      107       +1     
============================================
  Files            15       15              
  Lines          1075     1077       +2     
  Branches        120      122       +2     
============================================
  Hits            471      471              
  Misses          545      545              
- Partials         59       61       +2     

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