docs: fix references

Author: iampratik13

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

@@ -145,6 +145,10 @@ console.log( v );
 
 [arithmetic-mean]: https://en.wikipedia.org/wiki/Arithmetic_mean
 
+[@welford:1962a]: https://doi.org/10.1080/00401706.1962.10490022
+
+[@vanreeken:1968a]: https://doi.org/10.1145/362929.362961
+
 </section>
 
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