Update changelog a bit more

Author: A5rocks

newsfragments/3403.deprecated.rst

@@ -2,5 +2,5 @@ Trying to use absolute deadlines on a `trio.CancelScope` constructed
 with a relative deadline now raises, after being deprecated since Trio
 0.27.0.
 
-To move over, replace the `trio.CancelScope` object instead of mutating
-it.
+To switch a cancel scope from being relative to absolute, replace the
+`trio.CancelScope` object instead of trying to mutate it.