Fix undefined 'notebook' parameter in azpapermill_iterator template

[Manya Sharma (ManyaS-Git)]

Summary

.ci/steps/azpapermill_iterator.yml references ${{parameters.notebook}} (singular) in its inline script, but the template declares notebooks: (plural) in its own parameters: block, and the only caller (.ci/steps/ADOTrainDeployAMLJob.yml) passes notebooks:.

Referencing an undeclared template parameter makes Azure Pipelines template expansion fail, so this template cannot compile. The fix aligns the echo statement with the rest of the script, which already loops over ${{parameters.notebooks}}.

Verification

• The sibling template .ci/steps/azpapermill.yml declares notebook: (singular) and correctly uses ${{parameters.notebook}} everywhere — the plural/singular split here is purely a copy/paste artifact.
• Bug confirmed present on upstream microsoft/AI master.
• File still parses as valid YAML.

[Arsh Verma (ArshVermaGit)]

Looks good. The parameter mismatch is correctly fixed, and the verification confirms the template remains valid. Approving.