Fix the class name of a sample

[vjrantal]

Without the fix, running this sample would fail to an error like:
Could not find or load main class

Checklist

• I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-java/blob/master/.github/CONTRIBUTING.md).
• I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • I edited the corresponding document in the devdoc folder and added or modified requirements.
• I submitted this PR against the correct branch:
  • This pull-request is submitted against the master branch.

Reference/Link to the issue solved with this PR (if any)

Description of the problem

To reproduce without the fix:

cd azure-iot-sdk-java/device/iot-device-samples/transportclient-sample
mvn install -DskipTests
java -jar ~/.m2/repository/com/microsoft/azure/sdk/iot/samples/device/transportclient-sample/1.9.0/transportclient-sample-1.9.0-with-deps.jar
Error: Could not find or load main class samples.com.microsoft.azure.sdk.iot.MultiplexingSample

Description of the solution

The solution is to fix the class name to match what is defined in the source file.

[timtay-microsoft]

Looks good to me. I'm looking to get this merged in now.

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