AutoMLMonitor example class does not compile

### Type of issue

Code doesn't work

### Description

[Enter feedback here]
In the example code for AutoMLMonitor on [this help page](https://learn.microsoft.com/en-us/dotnet/machine-learning/how-to-guides/how-to-use-the-automl-api) under the section Configure Experiment Modelling, the code does not compile and shows
an error at this line:
`public IEnumerable GetCompletedTrials() => _completedTrials;`

"The name '_completedTrials' does not exist in the current context.

### Page URL

https://learn.microsoft.com/en-us/dotnet/machine-learning/how-to-guides/how-to-use-the-automl-api

### Content source URL

https://github.com/dotnet/docs/blob/main/docs/machine-learning/how-to-guides/how-to-use-the-automl-api.md

### Document Version Independent Id

80ed0d4d-63d3-49f0-ae42-d1f132137352

### Article author

@luisquintanilla

### Metadata

* ID: c3bb37d9-1c4d-5d5f-2ed6-2ef77f2cd627
* Service: **dotnet-ml**