Update documentation for Windows pub cache location to from APPDATA to LOCALAPPDATA

[jmagman]

dart-lang/pub#2297 changed the default pub cache path on Windows to %LOCALAPPDATA%\Pub\Cache
Update documentation still referring to %APPDATA%:

• https://dart.dev/tools/pub/environment-variables
• https://dart.dev/tools/pub/cmd/pub-get#the-system-package-cache

https://github.com/dart-lang/site-www/search?q=APPDATA&unscoped_q=APPDATA

[kwalrath]

Thanks, @jmagman! Do you want to fix this, or shall I open it up to the community?

[jmagman]

@kwalrath #2527