dpkg-architecture: Move -L argument to the Commands --help output section

The -L argument is not an option, it's a full blown command.

debian/changelog

@@ -17,6 +17,7 @@ dpkg (1.17.10) UNRELEASED; urgency=low
   * Add a workaround to start-stop-daemon for bogus OpenVZ Linux kernels that
     prepend, instead of appending, the " (deleted)" marker in /proc/PID/exe.
     Closes: #731530
+  * Move dpkg-architecture -L argument to the Commands --help output section.
 
   [ Updated manpages translations ]
   * German (Helge Kreutzmann).

scripts/dpkg-architecture.pl

@@ -49,11 +49,11 @@ sub usage {
 'Options:
   -a<debian-arch>    set current Debian architecture.
   -t<gnu-system>     set current GNU system type.
-  -L                 list valid architectures.
   -f                 force flag (override variables set in environment).')
     . "\n\n" . _g(
 'Commands:
   -l                 list variables (default).
+  -L                 list valid architectures.
   -e<debian-arch>    compare with current Debian architecture.
   -i<arch-alias>     check if current Debian architecture is <arch-alias>.
   -q<variable>       prints only the value of <variable>.