fix whitespace

compiler/GHC/Cmm/CallConv.hs

@@ -241,7 +241,7 @@ realArgRegsCover platform
   possible combinations of register values. Instead, there are generic helper
   stack frames that use a call_info word that describes the active registers
   and the number of stack words used by the arguments of a call.
-  
+
   These helper stack frames are currently:
       - stg_ret_t:    return a tuple to the continuation at the top of
                           the stack