Fix comment of `section "Exceptions"`
Fix comment of `section "Exceptions"`

I changed comment from `{- .. -}` to `--`.
Because Haddock comment for `section "Exceptions"` [1] was broken.

[1]:
https://downloads.haskell.org/~ghc/latest/docs/html/libraries/ghc-prim-0
.5.0.0/GHC-Prim.html

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3083

ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc