Use the correct docs URL in the due dates sidebar widget

What does this MR do and why?

Fix a wrong docs URL that was edited almost 3 years ago 😄 !31634 (merged)

• Old one: https://docs.gitlab.com/ee/user/group/epics/index.html#start-date-and-due-date
• New one: https://docs.gitlab.com/ee/user/group/epics/manage_epics.html#start-and-due-date-inheritance

Screenshots or screen recordings

This is about fixing the Learn more URL in the due date widget

MR acceptance checklist

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