doc: fix typo

[well-balanced]

  const stringValue =
    typeof value === 'object' ? 'j:' + JSON.stringify(value) : String(value)
  ...
  // String() is not necessary
  res.setHeader('Set-Cookie', serialize(name, String(stringValue), options))

stringValue has already been converted to string type from the earlier code line. therefore this code line is useless.

Bug

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Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
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Documentation / Examples

• Make sure the linting passes

[timneutkens]

Good catch!