[flutter_svg] Update README with an example to scale images

[androidseb]

Following up on my original PR https://github.com/dnfield/flutter_svg/pull/991 after the suggestion from @darshankawar here.

This is an update to the flutter_svg documentation to include an example of how to scale an SVG image without losing quality, as this is something currently missing from the documentation and causing some confusion.

I believe it resolves the following issues (there might be more since then, the original PR is from 2023, I haven't searched for more):

• https://github.com/dnfield/flutter_svg/issues/858
• https://github.com/yang-f/flutter_svg_provider/issues/47

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[androidseb]

The Gemini Code Assist review concern is valid, please address

Thank you, I've fixed my examples. My understanding is that these changes are exempt from requiring tests since they affect the docs and examples?

Do I need to make changes to the pubspec.yaml or changelog? I refrained from making changes there to avoid running into unnecessary conflicts for now.