Update ProvisioningX509Sample.java

[gjswalling]

point out where to add the signer/intermediate cert as in the readme "How to run the samples" step 4 iii for group enrollment

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.

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Description of the solution

[timtay-microsoft]

/azp run SDL

[timtay-microsoft]

/azp run Java Canary

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[timtay-microsoft]

Thanks for the contribution!

[timtay-microsoft]

/azp run Java Canary Basic

[timtay-microsoft]

/azp run Java Prod Basic

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).