docs(GlKeysetPagination): Fix markdown link

What does this MR do?

docs(GlKeysetPagination): Fix markdown link

Use actual footnote markdown syntax.

Before, the leading <sup> tag broke the parser, such that the anchor of the link caused the creation of a heading. Plus, the whole footnote was free text rather than wrapped in a <p> tag.

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

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