One use of "class" instead of "plugin" to refer to the `SharedPreferences` class specifically slipped through review. This updates it to make it clear that it's talking about a specific class, not the plugin.

Also updates the `setPrefix:` doc comment to clarify that it only applies to that class, now that the addition of other classes to the API surface has made that ambiguous.

Fixes https://github.com/flutter/flutter/issues/158404