Fix expression for element size's maximum value

[mjbshaw]

No description provided.

[robUx4]

This does not render the exponent correctly when run through mmark and xml2rfc.

Before the patch in RFC 8794 Section 6.3 we have:

with an octet length of 8 is able to express a size of 2⁽⁵⁶⁾-2 or 72,057,594,037,927,934 octets (or about 72 petabytes)

When this patch is applied we have:

with an octet length of 8 is able to express a size of 2^(56)-2 or 72,057,594,037,927,934 octets (or about 72 petabytes)

The <sup> is gone.

This would actually be a revert of a78b551 which fixed this.

[mjbshaw]

Thanks for the review. Sorry, I didn't realize this was an extension in mmark.