Fix withFileContents documentation

The documentation for `Distribution.Utils.Generic.withFileContents` is incorrect:

    $ cabal repl Cabal-syntax
    ghci> import Distribution.Utils.Generic
    ghci> withFileContents "README.md" $ \contents -> pure ("foo" `isInfixOf` contents)
    *** Exception: README.md: hGetContents: illegal operation (delayed read on closed handle)

Cabal-syntax/src/Distribution/Utils/Generic.hs

@@ -153,8 +153,8 @@ wrapLine width = wrap 0 []
 
 -- | Gets the contents of a file, but guarantee that it gets closed.
 --
--- The file is read lazily but if it is not fully consumed by the action then
--- the remaining input is truncated and the file is closed.
+-- The file is read lazily; if it is not fully consumed by the action then an
+-- exception is thrown.
 withFileContents :: FilePath -> (String -> IO a) -> IO a
 withFileContents name action =
   withFile