Update 2-playground-tutorial.md (#204)

Simply adds text to say _what_ to set height and width too. Otherwise the user has to scroll down and find the completed source. For me, this was annoying because the previous instructions stood on their own.

Author: cfjedimaster

content/docs/en/getting-started/2-playground-tutorial.md

@@ -213,7 +213,7 @@ Use a `<ListView>` to show tasks below the input field.
 1. Delete all `<Label>` components from the `<GridLayout>` block.
 1. Configure the `<StackLayout>`.
     * Remove background color.
-    * Set width and height.
+    * Set width and height both to `100%`.
 1. Configure the `<GridLayout>`.
     * Set the grid to consist of two columns and one row.
     * Set the width of the grid to 100% so that it takes the entire width of the screen.