Fix Sphinx warning.

doc/references/index.rst

@@ -9,7 +9,7 @@ If you use the BOOMER algorithm in a scientific publication, we would appreciate
 
 **Learning Gradient Boosted Multi-label Classification Rules**
 
-The algorithm was first published in the following `paper <https://doi.org/10.1007/978-3-030-67664-3_8>`_. A preprint version is publicly available on `arxiv.org <https://arxiv.org/pdf/2006.13346.pdf>`__.
+The algorithm was first published in the following `paper <https://doi.org/10.1007/978-3-030-67664-3_8>`__. A preprint version is publicly available on `arxiv.org <https://arxiv.org/pdf/2006.13346.pdf>`__.
 
 *Michael Rapp, Eneldo Loza Mencía, Johannes Fürnkranz, Vu-Linh Nguyen and Eyke Hüllermeier. Learning Gradient Boosted Multi-label Classification Rules. In: Proceedings of the European Conference on Machine Learning and Knowledge Discovery (ECML-PKDD), pages 124-140, 2020, Springer.*
 
@@ -26,7 +26,7 @@ The algorithm was first published in the following `paper <https://doi.org/10.10
 
 **Gradient-based Label Binning in Multi-label Classification**
 
-Gradient-based label binning (GBLB), which is an extension to the original algorithm, was proposed in the following `paper <https://doi.org/10.1007/978-3-030-86523-8_28>`_. A preprint version is available on `arxiv.org <https://arxiv.org/pdf/2106.11690.pdf>`__.
+Gradient-based label binning (GBLB), which is an extension to the original algorithm, was proposed in the following `paper <https://doi.org/10.1007/978-3-030-86523-8_28>`__. A preprint version is available on `arxiv.org <https://arxiv.org/pdf/2106.11690.pdf>`__.
 
 *Michael Rapp, Eneldo Loza Mencía, Johannes Fürnkranz and Eyke Hüllermeier. Gradient-based Label Binning in Multi-label Classification. In: Proceedings of the European Conference on Machine Learning and Knowledge Discovery in Databases (ECML-PKDD), pages 462-477, 2021, Springer.*
 
@@ -43,7 +43,7 @@ Gradient-based label binning (GBLB), which is an extension to the original algor
 
 **BOOMER – An Algorithm for Learning Gradient Boosted Multi-label Classification Rules**
 
-A technical report on the implementation of the BOOMER algorithm was published for open access in the following `article <https://doi.org/10.1016/j.simpa.2021.100137>`_.
+A technical report on the implementation of the BOOMER algorithm was published for open access in the following `article <https://doi.org/10.1016/j.simpa.2021.100137>`__.
 
 *Michael Rapp. BOOMER – An Algorithm for Learning Gradient Boosted Multi-label Classification Rules. In: Software Impacts, page 100137, 2021, Elsevier.*