yampa: Adjust indentation to two spaces (KSG Haskell 1.3.0 - 4.2). Refs

#215.

Suggestion: Each time a new expression or block-like construct is opened, the
indent should increase by two spaces. When the section ends, the indent returns
to the previous indent level. The indent level applies to both Haskell
expressions and comments throughout the block.

yampa/src/FRP/Yampa/Diagnostics.hs

@@ -17,4 +17,4 @@ usrErr mn fn msg = error (mn ++ "." ++ fn ++ ": " ++ msg)
 -- | Reports an error in Yampa's implementation.
 intErr :: String -> String -> String -> a
 intErr mn fn msg = error ("[internal error] " ++ mn ++ "." ++ fn ++ ": "
-                          ++ msg)
+                           ++ msg)