Fix documentation for allocate_pages function

[splaled]

The documentation for allocate_pages here states that count is the number of bytes to allocate, but in the UEFI specification, it's the number of pages. It appears this function also uses it as count of pages.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)