obvious typo in doc: "loader: MiniCssExtractPlugin.loader", with loader missing

[webpack-bot]

Do you want to request a feature or report a bug?
clear and obvious bug

What is the current behavior?
https://webpack.js.org/plugins/mini-css-extract-plugin/
in the example, the json is invalid.
"loader: MiniCssExtractPlugin.loader", with loader missing
This is quite obvious.

If the current behavior is a bug, please provide the steps to reproduce.

What is the expected behavior?
fix doc.
If this is a feature request, what is motivation or use case for changing the behavior?

Please mention other relevant information such as the browser version, Node.js version, webpack version, and Operating System.

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This issue was moved from webpack/webpack#7596 by @montogeek. Original issue was by @MagicPoulp.