Fix payload_generator.format_payload rspec

The platform should match.

It was broken due to:
f51cbfffb8e75f31fd7dc7bdc11089289829edc3

No redmine ticket or PR for the above fix.

Error:

```
Msf::PayloadGenerator ...........................................*..................F.......
  1) Msf::PayloadGenerator#format_payload when format is an executable format applies the appropriate executable format
     Failure/Error: payload_generator.format_payload(shellcode)
       <Msf::Util::EXE (class)> received :to_executable_fmt with unexpected arguments
         expected: (#<Framework (0 sessions, 0 jobs, 0 plugins)>, "x86", "Windows", "PQXY", "exe", {:inject=>false, :template_path=>"/Users/wchen/rapid7/msf/data/templates", :template=>"template_x86_windows.exe"})
              got: (#<Framework (0 sessions, 0 jobs, 0 plugins)>, "x86", #<Msf::Module::PlatformList:0x007fee499f3ed8 @platforms=[Msf::Module::Platform::Windows]>, "PQXY", "exe", {:inject=>false, :template_path=>"/Users/wchen/rapid7/msf/data/templates", :template=>"template_x86_windows.exe"})
```