Added space to make ordered list format correctly in reduce-sum section (design-docs pull req #17)

Author: bbbales2

src/stan-users-guide/parallel-computing.Rmd

@@ -3,6 +3,7 @@
 Stan has two mechanisms for parallelizing calculations used in a model: `reduce_sum` and and `map_rect`.
 
 The main differences are:
+
 1. `reduce_sum` requires the result of the calculation to be a scalar, while `map_rect` returns a list of vectors
 2. `reduce_sum` has a more flexible interface and can accept arbitrary Stan types as arguments, `map_rect` is more restrictive on what arguments can be and how they are shaped
 3. `map_rect` can parallelize work over multiple computers or a single computer, while `reduce_sum` works only on a single computer
@@ -164,7 +165,7 @@ target += reduce_sum(reducer_func, y,
 ```
 
 `reduce_sum` automatically breaks the sum into roughly `grainsize` sized pieces
-and computes them in parallel. `grainsize = 0` specifies that the grainsize should
+and computes them in parallel. `grainsize = 1` specifies that the grainsize should
 be estimated automatically.
 
 ### Picking the Grainsize