docs: update npm-cache directory location

[jouni]

The location of the npm-cache/_libvips directory seems to be incorrect in the documentation. On my Windows 11 setup, it’s located under AppData/Local, not AppData/Roaming:

Whereas the AppData/Roaming directory doesn't have have an npm-cache directory at all:

[pieh]

I'm not on Windows right now, but googling npm cache location it seems depending on some factors (like used npm cli toggles) it can end up in either place, so maybe both should be mentioned?

[jouni]

Okay, thanks for checking that! Yeah, I suppose both could be mentioned.

[pieh]

Thanks!