debian/postinst: fix /dev/nill typo to /dev/null

debian/goatherd.postinst

@@ -8,7 +8,7 @@ if test "$1" = "configure"; then
         adduser --quiet --system --group --no-create-home --home $statedir goatherd
     fi
 
-    if ! dpkg-statoverride --list $statedir >/dev/nill; then
+    if ! dpkg-statoverride --list $statedir >/dev/null; then
         dpkg-statoverride --quiet --update --add goatherd goatherd 0700 $statedir
     fi
 fi