bump haddock submodule
bump haddock submodule

Summary:
ae2c9b40f5b6bf272251d1f4107c60003f541b62 introduced some changes that
broke haddock, [1] fixed them and this patch bumps the haddock submodule
to include the fixes.

[1]: https://github.com/haskell/haddock/pull/970

Test Plan: build haddock (make or hadrian)

Reviewers: bgamari, AndreasK

Reviewed By: AndreasK

Subscribers: osa1, AndreasK, rwbarton, carter

GHC Trac Issues: #15900

Differential Revision: https://phabricator.haskell.org/D5341

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc