docs: fix formatting on signature-transfer.md

[0xdevant]

This PR is to fix the markdown syntax for signature-transfer.md on Permit2.

Previously the double asterisk(2**248) accidentally bolded 248 instead of properly displaying it as 2 to the power of 248.

[vercel[bot]]

@0xdevant is attempting to deploy a commit to the Uniswap Team on Vercel.

A member of the Team first needs to authorize it.