Fix typo in arch string description

[Millak]

No description provided.

[mewmew]

Thanks for the PR @Millak.

I think lower-case i386 is also in use? At least according to Wiki. From https://en.wikipedia.org/wiki/I386:

The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985

[Millak]

That was my first thought, but then I saw http://msdn.microsoft.com/en-us/gg463119.aspx from pe.go which has IMAGE_FILE_MACHINE_I386 and I wasn't sure if you used AMD64 and IA64 because those ones are normally capitalized or because that's how Microsoft's documentation has it.