i18n docs have out of date info on file generation

[drrnbrns]

The docs at https://docs.flutter.dev/ui/accessibility-and-internationalization/internationalization are excellent and clear but have an issue in step 6. Where it says "You should find generated files in the directory at the path you specified with the arb-dir or output-dir options", apparently that is obsolete.

I spent some time trying to figure out what I'd done wrong, but apparently they are now under ".dart_tools/flutter_gen/gen_l10n", so the document should be updated to reflect that. Others have posted about this at https://stackoverflow.com/a/65192314/2740650.