Small stylistic error/typo fix in the docs for adding your own library

[norbertcygiert]

Description

Fix the small stylistic mistake in the documentation.

Type of change

• Bug fix (non-breaking change which fixes an issue) (docs fix)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there