fix typo in helper name (kebab case, not snake)

Author: donoghuc

.buildkite/pull_request_pipeline.yml

@@ -142,7 +142,7 @@ steps:
 
       docker build -t test-runner-image -f x-pack/distributions/internal/observabilitySRE/docker/Dockerfile .
       docker run test-runner-image /bin/bash -c '
-        source ci/test_split_utils.sh
+        source ci/get-test-half.sh
         specs=$(get_test_half 0)
         ./gradlew --info --stacktrace -PrunTestsInFIPSMode=true runIntegrationTests -PrubyIntegrationSpecs="$specs"
       '
@@ -164,7 +164,7 @@ steps:
 
       docker build -t test-runner-image -f x-pack/distributions/internal/observabilitySRE/docker/Dockerfile .
       docker run test-runner-image /bin/bash -c '
-        source ci/test_split_utils.sh
+        source ci/get-test-half.sh
         specs=$(get_test_half 1)
         ./gradlew --info --stacktrace -PrunTestsInFIPSMode=true runIntegrationTests -PrubyIntegrationSpecs="$specs"
       '