chore(drag-drop): fix incorrect property description

[crisbeto]

Fixes a wrong docstring that was copied from another property.

[jelbourn]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: tests_saucelabs" is failing
     status "ci/circleci: bazel_build_test" is pending
     status "ci/circleci: build_devapp_aot" is pending
     status "ci/circleci: tests_browserstack" is pending
     status "ci/circleci: tests_local_browsers" is pending

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