update copyright year

Author: yihui

exampleSite/config.toml

@@ -33,7 +33,7 @@ footnotereturnlinkcontents = "↩"
 
 [params]
     description = "A website built through Hugo and blogdown."
-    footer = "© [Yihui Xie](https://yihui.org) 2017 -- 2019 | [Github](https://github.com/yihui) | [Twitter](https://twitter.com/xieyihui)"
+    footer = "© [Yihui Xie](https://yihui.org) 2017 -- 2020 | [Github](https://github.com/yihui) | [Twitter](https://twitter.com/xieyihui)"
 
 [markup]
   [markup.goldmark]