Use posterdown_ids

Change-Id: I87be1c2ba983553db782f009f26a52d7eb89ece0
diff --git a/R/poster.Rmd b/R/poster.Rmd
index 677a042..be15f0b 100644
--- a/R/poster.Rmd
+++ b/R/poster.Rmd
@@ -68,7 +68,7 @@
 author_textsize: "32pt"
 
 output:
-  posterdown::posterdown_html
+  posterdown::posterdown_ids
 ---
 
 ```{r setup, include=FALSE, echo=FALSE, warning=FALSE}