Fix typo in TcErrors.hs
Fix typo in TcErrors.hs

This fixes a very simple typo in TcErrors.hs. I hope this is small
enough in scope to be accepted through GitHub.

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