Fix comment (broken link to users_guide) in $(TOP)/
Fix comment (broken link to users_guide) in $(TOP)/

There was a broken link to users_guide at a comment.

* compiler/types/InstEnv.hs
  - type-class-extensions.html -> glasgow_exts.html

Reviewers: bgamari, austin

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3092

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