docs: remove excess backslash escapes

[textbook]

These had previously been introduced for *.mdx files (see #2917) but seem to no longer be needed. As a result, the rendered output includes redundant escapes that would break file matching, e.g.:

module.exports = {
  module: {
    rules: [
      { test: /\\.css$/, use: 'css-loader' },
      { test: /\\.ts$/, use: 'ts-loader' }
    ]
  }
};

See e.g. this Stack Overflow question for resulting confusion.

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[chenxsan]

Thank you for taking your time, seems like mdx fixed the bug this year mdx-js/mdx#991 and we failed to update those changes.

[chenxsan]

@textbook Thanks for your contribution to webpack documentation.