Update pretty printing to use 2 space indent.

This is inline with all the documentation and examples I've seen throughout the cabal docs, as well as all cabal file templates I've seen in the wild and those constructed with stack and summoner (the two other leading ways to initialize a project). It appears that `cabal format` hasn't been released yet so this is unlikely to cause a large number of diffs.

/cc @phadej who made the original change.

Cabal/Distribution/Pretty.hs

@@ -92,4 +92,4 @@ lines_ s  =
 
 -- | the indentation used for pretty printing
 indentWith :: Int
-indentWith = 4
+indentWith = 2