Fix minor formatting issue in cpp11 vignette

[amoeba]

I found a very minor thing in the cp11 vignette in the Missing values section. Markdown renders need lists to have a preceding blank line in order to render properly.

Before this change, the corresponding text rendered like:

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).

After:

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).

I made a guess the current rendering wasn't intentional but feel free to close if it was.

[romainfrancois]

Thanks.