Include path for SVG files in Flag icons is incorrect
This is similar to #20 but for the Flag icons.

The path defined here: https://github.com/coreui/coreui-icons/blob/master/scss/flag/_core.scss#L14 isn't quite right because the `svg` folder is actually one level deeper.  This error was emitted when transpiling the SCSS:

>Syntax Error: ModuleNotFoundError: Module not found: Error: Can't resolve '../svg/flag/cif-ad.svg' in '/project/node_modules/@coreui/icons/scss/flag'

To fix this, I changed this line:

`background-image: url(../svg/flag/#{$coreui-icons-prefix}#{$icon}.svg);`

to

`background-image: url(../../svg/flag/#{$coreui-icons-prefix}#{$icon}.svg);`