Fixes typo in generating X64 PE file

[msutovsky-r7]

This fixes a typo in msf/util/exe.rb:

def self.to_executable(framework, arch, plat, code = '', opts = {})
     [...]
      if plat.index(Msf::Module::Platform::Windows)
             return to_win32pe(framework, code, opts)  if arch.index(ARCH_X86)
             return to_win64pe(framework, code, opts)  if arch.index(ARCH_X86)

This bug was causing failure, when generating Windows x64 payloads.

[dledda-r7]

lgtm, waiting CI