Update gwb-grid_app.md

Updates the description of n_cell to be "number of cells".

Includes other edits for readability.

doc/sphinx/user_manual/how_to_use_the_applications/gwb-grid_app.md

@@ -2,9 +2,9 @@
 The gwb-grid app
 ================
 
-While the `gwb-dat` app is great at providing individual points, providing gridded output can be very useful as well, especially when you want to visualize the result. This is exactly what the `gwb-grid` app does. It creates a grid for you with the values you request and outputs it as a `vtu` file. Using `gwb-grid` is very similar to using `gwb-dat`: `gwb-grid world_builder_file.wb grid_info.grid`. The only difference is that you now have to provide a `.grid` file instead of a `.dat` file.
+While the `gwb-dat` app is great at providing individual points, providing gridded output can be very useful especially when visualizing the result. This is exactly what the `gwb-grid` app does. It creates a grid with the values requested and outputs it as a `vtu` file. Using `gwb-grid` is very similar to using `gwb-dat`: `gwb-grid world_builder_file.wb grid_info.grid`. The only difference is that you now provide a `.grid` file instead of a `.dat` file.
 
-A grid file consists of a number of parameters you can set. Each parameter should start with a name (no spaces) followed by an equal sign, followed by the value. The available parameters are:
+A grid file consists of a number of parameters. Each parameter should start with a name (no spaces) followed by an equal sign, followed by the value. The available parameters are:
 
 1. `grid_type`: Options are `cartesian`, `sphere`, `chunk` and `anullus`. 
 2. `dim`: dimension, either 2 or 3.
@@ -16,9 +16,9 @@ A grid file consists of a number of parameters you can set. Each parameter shoul
 8. `y_min`: either the maximum y value or maximum latitude.
 9. `z_min`: either the minimum z value or minimum radius.
 10. `z_min`: either the maximum z value or maximum radius.
-11. `n_cell_x`: either the cells in the x or longitude direction.
-12. `n_cell_y`: either the cells in the y or latitude direction.
-12. `n_cell_z`: either the cells in the z or radius direction.
+11. `n_cell_x`: either the number of cells in the x or longitude direction.
+12. `n_cell_y`: either the number of cells in the y or latitude direction.
+12. `n_cell_z`: either the number of cells in the z or radius direction.
 
 An example of a grid file is the following: 
 
@@ -29,4 +29,4 @@ An example of a grid file is the following:
 
 When you run it, it will produce a vtu file with the same name in the directory you run it from. 
 
-More examples can be found in the `tests/gwb-grid/` directory.
+More examples can be found in the `tests/gwb-grid/` directory.