Update activate-theme.ts to use themeFolderName. (#1119)

This PR update the documentation to reflect the current state.

## What problem is it solving?
It's broken.

## How is the problem addressed?
Updated it to mention the relevant parameter.

## Testing Instructions

packages/playground/blueprints/src/lib/steps/activate-theme.ts

@@ -8,8 +8,7 @@ import { StepHandler } from '.';
  * <code>
  * {
  * 		"step": "activateTheme",
- * 		"pluginName": "Storefront",
- * 		"pluginPath": "/wordpress/wp-content/themes/storefront"
+ * 		"themeFolderName": "storefront"
  * }
  * </code>
  */