Corrected variable name (#36666)

Variable created in the earlier step `pfiResults` does not match the one being used `pfi`

Author: nishanc

docs/machine-learning/how-to-guides/how-to-use-the-automl-api.md

@@ -720,6 +720,6 @@ To determine feature importance using AutoML:
 
     ```csharp
     var featureImportance = 
-        pfi.Select(x => Tuple.Create(x.Key, x.Value.Regression.RSquared))
+        pfiResults.Select(x => Tuple.Create(x.Key, x.Value.Regression.RSquared))
             .OrderByDescending(x => x.Item2);    
     ```