Update documentation for sass indented syntax

[lorefnon]

Current docs recommend use of following config to enable sass indented syntax:

                    {
                        loader: "sass-loader",
                        options: {
                            sassOptions: {
                                indentedSyntax: true
                            }
                        }
                    },

It looks like after a breaking change in sass-loader, we must use this option now:

                    {
                        loader: "sass-loader",
                        options: {
                            sassOptions: {
                                syntax: "indented"
                            }
                        }
                    },

Once we have a confirmation from sass-loader devs (webpack-contrib/sass-loader#1240) that this undocumented change was intentional, we can update the docs.