docs: Update css preprocessor story.

Author: Brocco

docs/documentation/stories/css-preprocessors.md

@@ -28,7 +28,7 @@ ng new sassy-project --style=sass
 Or set the default style on an existing project:
 
 ```bash
-ng set defaults.styleExt scss
+ng config defaults.styleExt scss
 ```
 
 Style strings added to the `@Component.styles` array _must be written in CSS_ because the CLI cannot apply a pre-processor to inline styles.