Update Documentation of Task Execution Properties

[arisen1145]

The property spring.task.execution.pool.max-threads documented in articel 42 is not the correct property. In the appendix and in code it is named spring.task.execution.pool.max-size

See:
https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-task-execution-scheduling

[philwebb]

Closing in favor of PR #18331