Update New-IscsiServerTarget.md

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During my testing, I found that the underscore character is not allowed in the target name. Attempting to include it results in the following error:

New-IscsiServerTarget : Unable to create the iSCSI target.

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• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
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[learn-build-service-prod]

Learn Build status updates of commit 3a982aa:

✅ Validation status: passed

File	Status	Preview URL	Details
docset/winserver2022-ps/iscsitarget/New-IscsiServerTarget.md	✅Succeeded	View (WindowsServer2022-ps)

For more details, please refer to the build report.

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[learn-build-service-prod]

Learn Build status updates of commit 49c41f7:

✅ Validation status: passed

File	Status	Preview URL	Details
docset/winserver2022-ps/iscsitarget/New-IscsiServerTarget.md	✅Succeeded	View (WindowsServer2022-ps)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel