Wrong import path separator used in readme example

[dhafinrayhan]

In the readme, the example code for importing the package is:

import 'material_symbols_icons\symbols.dart'

This is wrong, because Dart imports use slash (/) rather than backslash (\).

Thanks to user MendelG in stackoverflow.com for figuring out.

[timmaffett]

Great catch. It is also missing the 'package:' prefix. I merged you pull request and added the package: as well.

Thanks!