Ml-agents: Outdated use_curiosity still in documentation

[JPhilipp]

I understand "use_curiosity" is superseded by the "reward_signals: ... curiosity: ..." syntax in config yaml files, but it still gets mentioned in the Hallway example docs.

[AcelisWeaven]

This is not yet merged into master, by it seems it has been fixed in #3386 (10 minutes before your issue :) )

[chriselion]

Whoops, the PR was in response to @JPhilipp's forum post https://forum.unity.com/threads/does-use_curiosity-still-exist.824040/

I'll close this off, since it will be merged into master sometime this week (after the release is completed)