(AzureCXP) fixes MicrosoftDocs/azure-docs#99582

Updates the example for using tokencredential in .NET. The previous example was not clear - for using TelemetryConfig instance. Instead, the TelemetryConfig.Active would be used to update the connection string and token.

Author: AnuragSingh-MSFT

articles/azure-monitor/app/azure-ad-authentication.md

@@ -76,12 +76,9 @@ Application Insights .NET SDK supports the credential classes provided by [Azure
 Below is an example of manually creating and configuring a `TelemetryConfiguration` using .NET:
 
 ```csharp
-var config = new TelemetryConfiguration
-{
-    ConnectionString = "InstrumentationKey=00000000-0000-0000-0000-000000000000;IngestionEndpoint=https://xxxx.applicationinsights.azure.com/"
-}
+TelemetryConfiguration.Active.ConnectionString = "InstrumentationKey=00000000-0000-0000-0000-000000000000;IngestionEndpoint=https://xxxx.applicationinsights.azure.com/";
 var credential = new DefaultAzureCredential();
-config.SetAzureTokenCredential(credential);
+TelemetryConfiguration.Active.SetAzureTokenCredential(credential);
 ```
 
 Below is an example of configuring the `TelemetryConfiguration` using .NET Core: