spectral: enable exact-reals

The benchmark only needed a small update to the expected stdandard
output file (missing newline).

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: build & run

Reviewers: dfeuer, bgamari

Reviewed By: dfeuer

Differential Revision: https://phabricator.haskell.org/D3363