(v3.2.9) Environment registration field bug

[AlejandroCN7]

Description

There was a bug in a variable_names field for environments configuration files processing function. This PR solves this problem.

Motivation and Context

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)
• Improvement (of an existing feature)
• Others

Checklist:

• I've read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests.
• I have updated the documentation accordingly.
• I have reformatted the code using autopep8 second level aggressive.
• I have reformatted the code using isort.
• I have ensured cd docs && make spelling && make html pass (required if documentation has been updated.)
• I have ensured pytest tests/ -vv pass. (required).
• I have ensured pytype -d import-error sinergym/ pass. (required)

Changelog:

• Fix variables_names to variable_names in environment parameters creation function from environment configuration files.