[DOC] Complete unfilled placeholder in MoleculeLoader docstring

[NoorMajdoub]

Reference Issues/PRs

Fixes #392

What does this implement/fix? Explain your changes.

Completes the unfilled placeholder in the MoleculeLoader docstring in pyaptamer/data/loader.py by:

• Adding two concrete examples of string paths
• Adding clarification of rule that str paths must follow

Did you add any tests for the change?

No ,documentation-only change.

Any other comments?

-Documentation-only change (no logic or test changes needed).
-No new dependencies

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files