patch applied (ghc): fix ref to utils/ext-core, which moved to Hackage (extcore package)

[Simon Marlow]

Wed Dec  1 01:21:47 PST 2010  Simon Marlow <marlowsd at gmail.com>
  * fix ref to utils/ext-core, which moved to Hackage (extcore package)

    M ./docs/users_guide/using.xml -3 +2

View patch online:
http://darcs.haskell.org/cgi-bin/darcsweb.cgi?r=ghc;a=darcs_commitdiff;h=20101201092147-12142-df9f230cba37e396d8c079e35cbdc9482450f766.gz