fix(drag-drop): Use correct drop event

[pas2al]

Fixes the drop event name in the drag-drop demo. See #12086

[crisbeto]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "continuous-integration/travis-ci/pr" is failing

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[pas2al]

How do we get travis green?

[crisbeto]

Looks like it was a flake.

[pas2al]

Fixed in #12104

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