Fix warning output for deprecated code path.

Author: ioquatix

lib/async/task.rb

@@ -136,7 +136,7 @@ def to_s
 
 		# @deprecated Prefer {Kernel#sleep} except when compatibility with `stable-v1` is required.
 		def sleep(duration = nil)
-			warn("`Async::Task#sleep` is deprecated, use `Kernel#sleep` instead.", uplevel: 1, category: :deprecated) if $VERBOSE
+			Kernel.warn("`Async::Task#sleep` is deprecated, use `Kernel#sleep` instead.", uplevel: 1, category: :deprecated) if $VERBOSE
 
 			super
 		end