Flip /: in documented default value of DPkg::Path

The value as shown in the NEWS file (not in the code) has a typo in
which just a "/" and ":" are swapped.

Closes: #917986

debian/NEWS

@@ -1,7 +1,7 @@
 apt (1.8.0~alpha3) unstable; urgency=medium
 
   The PATH for running dpkg is now configured by the option DPkg::Path,
-  and defaults to "/usr/sbin:/usr/bin:/sbin/:bin". Previous behavior of
+  and defaults to "/usr/sbin:/usr/bin:/sbin:/bin". Previous behavior of
   not changing PATH may be restored by setting the option to an empty string.
 
   Support for /etc/apt/auth.conf.d/ has been added, see apt_auth.conf(5).