Summary:
`core-spec.pdf` was emitting parse errors due to not specifying
role arguments in some uses of `nth`. This patch adds those
role arguments. (Credit goes to Richard Eisenberg for actually
figuring out what said arguments should be.)

Test Plan: Read it

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15373

Differential Revision: https://phabricator.haskell.org/D4965