Fix minor formatting issue in cpp11 vignette (#309)

Markdown renders need lists to have a preceding blank line in order
to render properly

Author: amoeba

vignettes/cpp11.Rmd

@@ -527,6 +527,7 @@ doubles attribs() {
 
 ## Missing values {#na}
 If you're working with missing values, you need to know two things:
+
 * How R's missing values behave in C++'s scalars (e.g., `double`).
 * How to get and set missing values in vectors (e.g., `doubles`).