[xml/en] copy edit and fix code typo

under well-formedness and validation, the first line of comment within code blocks should be written as text paragraph instead, otherwise anyone who simply copy-paste the code will not be able to preview the result, the original code block is now separated into three parts, each preceded by a single line of text paragraph for clarity, also the external dtd file had a typo in last commit (60a1a43) that should have been removed at the end

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