Docs: fix nonexistent timeout key names in Custom executor

What does this MR do and why?

The Terminating and killing executables section names two configuration keys, exec_terminate_timeout and exec_force_kill_timeout, that do not exist. The Custom executor keys are graceful_kill_timeout and force_kill_timeout — used in the config example earlier on this same page, defined in common/config.go, and listed in the advanced configuration reference that this section already links to. A reader who copies the prose key names into config.toml gets a setting that is silently ignored.

The incorrect names have been present since the page was introduced in 6eadf2f8d (2019), where the config example on the same page already used the correct keys. A pickaxe search across history confirms the exec_terminate_timeout/exec_force_kill_timeout strings have only ever appeared in docs, never in the code.

The docs-locale/{fr-fr,ko-kr,ja-jp}/executors/custom.md copies carry the same strings; they re-sync from the English source, so this MR edits only the English page.