Fix errors W3C Markup Validation

[h-enk]

Description

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Steps to reproduce

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• https://validator.w3.org/nu/?doc=https%3A%2F%2Fgetdoks.org%2F
• https://validator.w3.org/nu/?doc=https%3A%2F%2Fgetdoks.org%2Fdocs%2Fprologue%2Fintroduction%2F
• https://validator.w3.org/nu/?doc=https%3A%2F%2Fgetdoks.org%2Ftutorial%2Finstall-requirements%2F

Expected result

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Actual result

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Environment

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