docs: Windows port README should include make submodule as a build step

[mattytrentini]

Checks

• I agree to follow the MicroPython Code of Conduct to ensure a safe and respectful space for everyone.

• I've searched for existing issues and didn't find any that matched.

Documentation URL

https://github.com/micropython/micropython/blob/master/ports/windows/README.md

Description

The build steps for the Windows port are missing make submodule. Build errors occur because micropython-lib (specifically the ssl module) isn't found if that command isn't executed.