Old behaviour of *async in documentation

[qbojj]

First of all I do not know if this is a correct place to post this issue but:

it looks like old behaviour of of Executor::*async in the cookbook use the old return type:

• https://taskflow.github.io/taskflow/classtf_1_1Future.html claims that those functions return tf::Future
• [1] https://taskflow.github.io/taskflow/index.html#QuickStartLaunchAsyncTasks still uses tf::Future<optional<T>>
• (not in the website) https://github.com/taskflow/taskflow#launch-asynchronous-tasks uses the same code sample as [1]

probably the same problem occurs in other places but I didn't look for them too much.

[tsung-wei-huang]

@qbojj thank you for pointing this out! I will fix them. In the future, you can consider posting issues here. Thanks!