ParseXS: refactor: reindent Node::Sig::parse()

My previous merge commit moved (and renamed) this function from
ParseXS.pm to ParseXS/Node.pm. The former file has an indent of 2, while
the latter has an indent of 4. When I moved the code across, I forgot to
reindent it. So this commit does that now.

Whitespace-only change, apart from rewrapping some code comment blocks.