docs: minor cmake option name correction

[veprbl]

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[Copilot]

Pull request overview

This PR corrects a documentation error where the CMake option for building the ONNX plugin was incorrectly referenced as ACTS_PLUGIN_ONNX instead of the correct -DACTS_BUILD_PLUGIN_ONNX=ON.

• Corrects the CMake option name in the ONNX plugin documentation to match the actual option defined in CMakeLists.txt

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[acts-project-service]

🔴 Athena integration test results [2922783]

🔴 Some tests have failed!

Please investigate the pipeline!

status	job	report
🟢	report_pull_request
🟢	run_workflow_tests_run4_mc
🟢	run_workflow_tests_run2_mc
🟢	run_workflow_tests_run2_data
🟢	run_workflow_tests_run3_mc
🟢	run_workflow_tests_run3_data
🟢	test_ActsPersistifyEDM
🟢	test_ActsDumpGeometryIdentifiers
🔴	test_ActsCheckObjectCountsWorkflowHgtd
🔴	test_ActsCheckObjectCountsCachedWorkflow
🔴	test_ActsCheckObjectCountsLegacyWorkflow
🔴	test_ActsCheckObjectCountsWorkflow
🔴	test_ActsEFTrackFit
🟢	test_ActsPersistifySeeds
🟢	test_ActsBenchmarkWithSpot
🟢	test_ActsAnalogueClustering
🟢	test_ActsWorkflowHeavyIons
🟢	test_ActsWorkflowFastTracking
🟢	test_ActsWorkflowCached
🟢	test_ActsWorkflow
🟢	test_ActsValidateAmbiguityResolution
🟢	test_ActsValidateResolvedTracks
🟢	test_ActsValidateTracks
🟢	test_ActsValidateActsCoreSpacePoints
🟢	test_ActsValidateActsSpacePoints
🟢	test_ActsValidateSeeds
🟢	test_ActsValidateOrthogonalSeeds
🟢	test_ActsValidateClusters
🟢	test_ActsGx2fRefitting
🟢	test_ActsGSFRefitting
🟢	test_ActsKfRefitting
🟢	test_ActsExtrapolationAlgTest
🟢	test_ActsITkTest