Fix formating of code block in vectors.rst (facebookincubator#11163)

Summary:
In vectors.rst, there is a block of code that is not properly formatted.

Pull Request resolved: facebookincubator#11163

Reviewed By: kagamiori

Differential Revision: D64224262

Pulled By: kgpai

fbshipit-source-id: 401b948e01739b4ee5025fac11af02e2488444ba

velox/docs/develop/vectors.rst

@@ -130,7 +130,9 @@ of rows in the vector.
 
 Vectors are always held by std::shared_ptr using the VectorPtr alias.
 
-using VectorPtr = std::shared_ptr<BaseVector>;
+.. code-block:: c++
+
+    using VectorPtr = std::shared_ptr<BaseVector>;
 
 The “bits” namespace contains a number of convenience functions for working with
 a nulls buffer.