Minor fix about markdown syntax on cpp11.Rmd (#306)

Author: yutannihilation

vignettes/cpp11.Rmd

@@ -529,6 +529,7 @@ doubles attribs() {
 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`).
 
 ### Scalars