UPDATED: electron-windows-store command line (#18499)

The `flatten` option has been removed from electron-windows-store dcc654df2e so running the command would produce an error:

> error: unknwon option --flatten

docs/tutorial/windows-store-guide.md

@@ -83,7 +83,6 @@ and output directories, the app's name and version, and confirmation that
 electron-windows-store `
     --input-directory C:\myelectronapp `
     --output-directory C:\output\myelectronapp `
-    --flatten true `
     --package-version 1.0.0.0 `
     --package-name myelectronapp
 ```