revert heading change

Author: EricWrightAtWork

articles/machine-learning/how-to-configure-auto-train.md

@@ -482,7 +482,7 @@ az ml job show -n $run_id --web
 
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-### Multiple child runs on a cluster
+### Multiple child runs on clusters
 
 Automated ML experiment child runs can be performed on a cluster that is already running another experiment. However, the timing depends on how many nodes the cluster has, and if those nodes are available to run a different experiment.