DOCS: correct description of top_k parameter in function docstring

[randyshee]

Thank you so much for sharing the code for DESP, and congratulations on the acceptance to NeurIPS 2024!

While exploring the package, I noticed that the top_k parameter was described as "The number of top routes to keep in the final solution" in desp/DESP.py, but it seems to refer to the number of top building blocks to consider for each bimolecular reaction. To clarify, I’ve updated the docstring to reflect this.

Additionally, I believe an enhancement that allows for returning more than just the top 1 route in the final solution (as the original description suggests) could be a valuable feature to consider for future updates.

Thanks again for the great work!