Add clarification that the code is of x86 32bit

[harryhanYuhao]

As the system call opecode for 32-bit and 64-bit linux are different, may be it is helpful to add explicit clarification that the code in tutorial is of 32 bit, and can not be assembled by nasm to elf64 format.

[q2dg]

In fact, in 2024 these tutorials should be modified to support x64 architecture the sooner the better.