Update reference.rst

Author: Evolutionary-Intelligence

docs/sphinx/source/reference.rst

@@ -15,6 +15,7 @@ Critical Papers to Some Metaphors-Based Optimization
 * 2020: `Nature inspired optimization algorithms or simply variations
   of metaheuristics?
   <https://link.springer.com/article/10.1007/s10462-020-09893-8>`_
+  https://doi.org/10.1007/s10462-020-09893-8
 * 2020: `Benchmarking in optimization: Best practice and open issues
   <https://arxiv.org/abs/2007.03488>`_
   https://doi.org/10.48550/arXiv.2007.03488