fix customized ElasticPoolUpdate model as well

[payiAzure]

Description

Fix the Management.Sql customized ElasticPoolUpdate Model as well. (Same issue with previous PR #4206. Related Rest spec PR: Azure/azure-rest-api-specs#2734)

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[payiAzure]

@dsgouda Hi Deepak, Can you please review this as well? I forgot to make changes in another file for the same issue we want to fix. Sorry about inconvenience.

[dsgouda]

LGTM subject to CI passing.