Update managed-identity.md to include Object ID

[gladjohn]

Update managed-identity.md to include Object ID

[learn-build-service-prod]

Learn Build status updates of commit 8bde705:

✅ Validation status: passed

File	Status	Preview URL	Details
msal-dotnet-articles/advanced/managed-identity.md	✅Succeeded	View

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[learn-build-service-prod]

Learn Build status updates of commit 8bde705:

❌ Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
	❌Error		Details

• [Error: GitBranchDeletedOrForcePushed]

Cannot sync git repo to specified commit because ref +refs/pull/142/merge has been deleted or has been force pushed.
 If this is pull request, please ensure it is mergeable (no merge conflict) and open (not closed).

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

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