Fixed typo

[roccivic]

Description

Fixed typo

Motivation and Context

• [ ] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (update in the documentation)

Checklist:

• [x] I've read the CONTRIBUTION guide (required)
• [x] I have updated the changelog accordingly (required).
• [ ] My change requires a change to the documentation.
• [ ] I have updated the tests accordingly (required for a bug fix or a new feature).
• [x] I have updated the documentation accordingly.
• [ ] I have ensured pytest and pytype both pass (by running make pytest and make type).

[Miffyli]

Thanks for the PR! Please make sure to complete all points in the checklist (changelog) :).

Merging this will have to wait till some CI problems have been addressed.

[roccivic]

You want an entry in the changelog for a one character spelling mistake fix? Or do you just want me to tick the box above?

[Miffyli]

You want an entry in the changelog for a one character spelling mistake fix? Or do you just want me to tick the box above?

An entry to changelog + updated contributors. A contribution is contribution, and all changes should be reflected in the changelog :)

[araffin]

Closing in favor of https://github.com/Stable-Baselines-Team/stable-baselines/pull/3 (will be merged soon)