Update metrics.rst (#3081)

docs/source/contrib/metrics.rst

@@ -29,7 +29,7 @@ Module :mod:`ignite.contrib.metrics.regression` provides implementations of
 metrics useful for regression tasks. Definitions of metrics are based on `Botchkarev 2018`_, page 30 "Appendix 2. Metrics mathematical definitions".
 
 .. _`Botchkarev 2018`:
-        https://arxiv.org/ftp/arxiv/papers/1809/1809.03006.pdf
+        https://arxiv.org/abs/1809.03006
 
 Complete list of metrics: