docs(concepts): remove npm from installation way (webpack#5919)

Author: behnammodi

src/content/concepts/index.mdx

@@ -132,7 +132,7 @@ In order to use a plugin, you need to `require()` it and add it to the `plugins`
 **webpack.config.js**
 
 ```javascript
-const HtmlWebpackPlugin = require('html-webpack-plugin'); //installed via npm
+const HtmlWebpackPlugin = require('html-webpack-plugin');
 const webpack = require('webpack'); //to access built-in plugins
 
 module.exports = {

src/content/concepts/plugins.mdx

@@ -50,7 +50,7 @@ Depending on how you are using webpack, there are multiple ways to use plugins.
 **webpack.config.js**
 
 ```javascript
-const HtmlWebpackPlugin = require('html-webpack-plugin'); //installed via npm
+const HtmlWebpackPlugin = require('html-webpack-plugin');
 const webpack = require('webpack'); //to access built-in plugins
 const path = require('path');