docs: fix broken Schubert & Gertz DOI link

Author: d-mahi14

lib/node_modules/@stdlib/stats/base/ndarray/variancech/README.md

@@ -199,7 +199,7 @@ console.log( v );
 
 [@chan:1983a]: https://doi.org/10.1080/00031305.1983.10483115
 
-[@schubert:2018a]: https://doi.org/10.1145/3221664.3221674
+[@schubert:2018a]: https://dl.acm.org/doi/10.1145/3221664.3221674
 
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