fix: regexp pattern

[tsctx]

> require("undici").parseMIMEType("text/plain; |charset=UTF-16; charset=UTF-8"); // before
{
  type: 'text',
  subtype: 'plain',
  parameters: Map(1) { 'charset' => 'UTF-16' },
  essence: 'text/plain'
}
> require("undici").parseMIMEType("text/plain; |charset=UTF-16; charset=UTF-8"); // after
{
  type: 'text',
  subtype: 'plain',
  parameters: Map(2) { '|charset' => 'UTF-16', 'charset' => 'UTF-8' },
  essence: 'text/plain'
}