Fix "Glasgow Haskell Compiler <release> Users Guide"
Fix "Glasgow Haskell Compiler <release> Users Guide"

If you go to
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/index.html
the window title has `<release>` in it.  I don't understand how it all
works, but inspired by the line below which produces a correct string in
the docs I just blindly changed it in the same way.  Cabal appears to
have the same problem.

Test Plan:
it'd be nice if I could check the result
on harbormaster, can I?

Reviewers: thomie, bgamari, austin

Reviewed By: bgamari

Subscribers: rwbarton, simonmar

Differential Revision: https://phabricator.haskell.org/D3458

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