[Datafactory] Update FactoryGitHubConfiguration discriminator value (#3643)

[LijuanZ]

This checklist is used to make sure that common issues in a pull request are addressed. This will expedite the process of getting your pull request merged and avoid extra work on your part to fix issues discovered during the review process.

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[LijuanZ]

@hvermis can you help take a look? Thanks!

[LijuanZ]

The bug is to fix issue: #3643. I put the issue number in the PR subject

[AutorestCI]

Automation for azure-sdk-for-python

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-python#2293

[azuresdkci]

Can one of the admins verify this patch?

[AutorestCI]

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Nothing to generate for azure-sdk-for-ruby

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The initial PR has been merged into your service PR:
Azure/azure-sdk-for-node#3520

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Azure/azure-sdk-for-go#2630

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Automation for azure-sdk-for-java

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-java#2136