fix link to gym create custom env doc

[arjun-krishna1]

Link to openai gym docs on creating custom environments was broken, changed it to openai gym docs on this which exists

Description

Change link to openai gym docs on creating custom envs as the old one was broken at https://stable-baselines.readthedocs.io/en/master/guide/custom_env.html

Motivation and Context

Fixes broken link in docs to one that works

• I have raised an issue to propose this change: Closes #1165

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)
• Documentation (update in the documentation)

Checklist:

• I've read the CONTRIBUTION guide (required)
• 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).
• I have updated the documentation accordingly.
• I have ensured pytest and pytype both pass (by running make pytest and make type).

[arjun-krishna1]

For issue #1165

[Miffyli]

Hey, please open this issue in the original stable-baselines repo (5% chance I forgot something). Also remember to complete the checklist (e.g., update the changelog) :)