fix syntax error

Author: chiral-carbon

examples/references.bib

@@ -246,7 +246,7 @@ @inproceedings{salakhutdinov2008bayesian
 
 @article{sidc2021sunspot,
   author={SILSO World Data Center},
-  address={Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium}
+  address={Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium},
   title={The International Sunspot Number},
   journal={International Sunspot Number Monthly Bulletin and online catalogue},
   adsurl={https://wwwbis.sidc.be/silso/datafiles}