dpkg: Update unpack.c header comment

The function is not huge any longer, just big, as it has been refactored
substantially over time. And although it can still be reduced further, it
is definitely not the defining trait of this file anymore.

src/unpack.c

@@ -1,6 +1,6 @@
 /*
  * dpkg - main program for package management
- * unpack.c - the huge function process_archive
+ * unpack.c - .deb archive unpacking
  *
  * Copyright © 1995 Ian Jackson <ijackson@chiark.greenend.org.uk>
  * Copyright © 2006-2015 Guillem Jover <guillem@debian.org>