remove schedule on already scheduled tasks

Author: IanButterworth

doc/src/manual/methods.md

@@ -636,12 +636,12 @@ julia> f(1)
 julia> g(1)
 "definition for Int"
 
-julia> fetch(schedule(t, 1))
+julia> fetch(t)
 "original definition"
 
 julia> t = Threads.@spawn f(wait()); yield();
 
-julia> fetch(schedule(t, 1))
+julia> fetch(t)
 "definition for Int"
 ```