Update reference.rst

Author: Evolutionary-Intelligence

docs/sphinx/source/reference.rst

@@ -12,6 +12,7 @@ Critical Papers to Some Metaphors-Based Optimization
   <https://onlinelibrary.wiley.com/doi/full/10.1111/itor.13176>`_
 * 2022: `A new taxonomy of global optimization algorithms
   <https://link.springer.com/article/10.1007/s11047-020-09820-4>`_
+  https://doi.org/10.1007/s11047-020-09820-4
 * 2020: `Nature inspired optimization algorithms or simply variations
   of metaheuristics?
   <https://link.springer.com/article/10.1007/s10462-020-09893-8>`_