fix: accidental Markdown usage in Nonce Schema section of permit2 docs

[aspiers]

The Nonce Schema section of the permit2 docs refers to the number 2 to the power of 248; however representing this as 2**248 twice within the same sentence inadvertently triggering Markdown's mechanism for bold text. So escape the ** characters to avoid this bold formatting.

Also fix a "this" / "thus" typo, and make a uint248 monospace for consistency.