use updated CITATION info

Author: njtierney

inst/CITATION

@@ -1,8 +1,8 @@
 citHeader("To cite greta in publications use:")
 
-citEntry(entry = "Article",
+bibentry(entry = "Article",
   title        = "{greta}: simple and scalable statistical modelling in R",
-  author       = personList(as.person("Nick Golding")),
+  author       = c(as.person("Nick Golding")),
   journal      = "Journal of Open Source Software",
   year         = "2019",
   volume       = "4",