Fix one character typo in my previous PR for debconf module (#2899)

Small oops in the previous PR #2530 commit.

Instead of `diff_dict` it slipped through as `diffdict`.

Please merge and sorry.

system/debconf.py

@@ -164,7 +164,7 @@ def main():
         if module._diff:
             after = prev.copy()
             after.update(curr)
-            diffdict = {'before': prev, 'after': after}
+            diff_dict = {'before': prev, 'after': after}
         else:
             diff_dict = {}