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 https://github.com/onaio/fhir-gateway-extension/issues/61

Engineer Checklist

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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