Update CrudExamplesTests.cs

[RobinSoenen]

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Small change in the documentation:

var response = await client.IndexAsync(tweet, "my-tweet-index"); // <2>

To var response = await client.IndexAsync(tweet, (IndexName)"my-tweet-index"); // <2>

in order to avoid an ambiguous call:

error CS0121: The call is ambiguous between the following methods or properties: 'ElasticsearchClient.IndexAsync(TDocument, IndexName, CancellationToken)' and 'ElasticsearchClient.IndexAsync(TDocument, Id?, CancellationToken)'

[cla-checker-service[bot]]

💚 CLA has been signed

[RobinSoenen]

Contributor agreement signed in the meantime