README.MSVC.md: Update for SpiderMonkey-115.x

Hi,

This MR attempts to update the Windows/Visual Studio build instructions as we are now using SpiderMonkey-115.x in GJS, in which, the good news is that there is less manual intervention that is involved, and currently the GIT master branch builds without changes needed at this point of writing, on the latest Visual Studio 2019 release.

With blessings, thank you!