Fix missing htmlwidgets import in highcharter demo

Change-Id: If743b5cb422db9b1b809251fdf2746ab82a4af36
diff --git a/demo/highcharter-example.R b/demo/highcharter-example.R
index e2947ea..b6167f7 100644
--- a/demo/highcharter-example.R
+++ b/demo/highcharter-example.R
@@ -1,4 +1,5 @@
 library(RKorAPClient)
+library(htmlwidgets)
 
 plotHighchart <- function(query = "Schlumpf",
                           years = c(2000:2010),