Fix test calling List.tail()

List.tail is not the same as List.tail() in Groovy

JAVA-3742

Author: jyemin

driver-core/src/test/functional/com/mongodb/operation/ChangeStreamOperationSpecification.groovy

@@ -525,7 +525,7 @@ class ChangeStreamOperationSpecification extends OperationFunctionalSpecificatio
 
         operation.startAtOperationTime(result.last().getTimestamp('clusterTime'))
         cursor = execute(operation, async)
-        result = nextAndClean(cursor, async, expected.tail.size())
+        result = nextAndClean(cursor, async, expected.tail().size())
 
         then:
         result == expected.tail()