Fix yaml syntax for threshold with warning_range (#383)

Author: xlamorlette-datadog

.gitlab/bp-runner.fail-on-breach.yml

@@ -13,5 +13,5 @@ experiments:
               - throughput > 3700 op/s
           - name: normal_operation/only-tracing
             thresholds:
-              - agg_http_req_duration_p50 < 0.116 ms
+              - threshold: agg_http_req_duration_p50 < 0.116 ms
                 warning_range: 2