Fixed incorrect reference in DistributionsAD extension #530

Job	Run time
test (1.6, ubuntu-latest, x64, ForwardDiff)	3m 27s
test (1.6, ubuntu-latest, x64, Tracker)	3m 33s
test (1.6, ubuntu-latest, x64, ReverseDiff)	3m 39s
test (1.6, ubuntu-latest, x64, Zygote)	4m 36s
test (1.6, macOS-latest, x64, ForwardDiff)	3m 52s
test (1.6, macOS-latest, x64, Tracker)	4m 54s
test (1.6, macOS-latest, x64, ReverseDiff)	4m 25s
test (1.6, macOS-latest, x64, Zygote)	5m 6s
test (1, ubuntu-latest, x64, ForwardDiff)	3m 50s
test (1, ubuntu-latest, x64, Tracker)	3m 45s
test (1, ubuntu-latest, x64, ReverseDiff)	4m 15s
test (1, ubuntu-latest, x64, Zygote)	5m 2s
test (1, macOS-latest, x64, ForwardDiff)	5m 59s
test (1, macOS-latest, x64, Tracker)	4m 0s
test (1, macOS-latest, x64, ReverseDiff)	5m 42s
test (1, macOS-latest, x64, Zygote)	5m 25s
	1h 11m 30s

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