fix(theme-default): Capitalise the 'P' in 'Next page'

[flother]

Summary

In the previous/next page links at the bottom of docs pages, the text used to read "Previous Page" (title case) and "Next page" (sentence case). This changes the "Next Page" link to title case so that the two links match style.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Timeless0911]

thanks