Bug in users manual, 6.4.2, section on pattern guards

[Matthew Cox]

One example given in this discussion is:

clunky env var1 var1 = case lookup env var1 of
  Nothing -> fail
  Just val1 -> case lookup env var2 of
    Nothing -> fail
    Just val2 -> val1 + val2
where
  fail = val1 + val2

this needs to be fixed to:

clunky env var1 var1 = case lookup env var1 of
  Nothing -> fail
  Just val1 -> case lookup env var2 of
    Nothing -> fail
    Just val2 -> val1 + val2
where
  fail = var1 + var2   <<<< (Added for emphasis)

I didn't see it myself on the first readthrough, but I was comparing structure between two examples
and noticed the discrepency. Let's fix it for correctness' sake.