fix: fix changed file path in docs

[rrsettgast]

PR #3372 introduced a warning/error in the readthedocs sphinx docs:

/home/docs/checkouts/readthedocs.org/user_builds/geosx-geosx/checkouts/develop/src/docs/sphinx/developerGuide/Contributing/UnitTests.rst:14: WARNING: Include file '/home/docs/checkouts/readthedocs.org/user_builds/geosx-geosx/checkouts/develop/src/coreComponents/unitTests/fluidFlowTests/testSinglePhaseBaseKernels.cpp' not found or reading it failed [docutils]

This PR seeks to fix the readthedocs builds.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.58%. Comparing base (ef14215) to head (24759ac).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3435   +/-   ##
========================================
  Coverage    57.58%   57.58%           
========================================
  Files         1105     1105           
  Lines        97797    97797           
========================================
  Hits         56318    56318           
  Misses       41479    41479           

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