[JENKINS-64738] Fixed reference to get the amiType class name

[TheImplementer]

Turns out the typo fixed in 608 wasn't the only thing wrong. The instance var is only reassigned inside the dropdownListBlock element, so instance.amiType should be used in the staplerClass attribute instead.

The fix has been tested manually and now works as expected.

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[TheImplementer]

Anyone who can review this? @res0nance maybe? It would be good to have it in the next release.