Update the example in Graphics.Vty

Previously this example had two problems which prevented a
new user copying the example out of haddock.

1) It unnecessarily relied on 'def', which comes from an
external package which the user might not know about,
and doesn't need to worry about.

2) Haddock wasn't rendering the back-ticks correctly, so
withForeColor appeared without the back-tick

3) It used the ScopedTypeVariables extension.

src/Graphics/Vty.hs

@@ -14,16 +14,19 @@
 -- See the vty-examples package for a number of examples.
 --
 -- @
+--  import "Graphics.Vty"
+--
 --  main = do
---      vty <- 'mkVty' def
---      let line0 = 'string' (def `withForeColor` 'green') \"first line\"
---          line1 = 'string' (def `withBackColor` 'blue') \"second line\"
+--      cfg <- 'standardIOConfig'
+--      vty <- 'mkVty' cfg
+--      let line0 = 'string' ('defAttr' ` 'withForeColor' ` 'green') \"first line\"
+--          line1 = 'string' ('defAttr' ` 'withBackColor' ` 'blue') \"second line\"
 --          img = line0 '<->' line1
 --          pic = 'picForImage' img
 --      'update' vty pic
---      e :: 'Event' <- 'nextEvent' vty
+--      e <- 'nextEvent' vty
 --      'shutdown' vty
---      print $ \"Last event was: \" ++ show e
+--      'print' (\"Last event was: \" '++' 'show' e)
 -- @
 -- 
 -- Good sources of documentation for terminal programming are: