Corrected attribute on uui-avatar example

[leekelleher]

Description

I'd noticed that the example of the <uui-avatar> component uses src instead of img-src.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Motivation and context

Following an example of how to use the <uui-avatar> component, I noticed that the README's example uses the src attribute, but that didn't work, and saw on Storybook that the attribute is img-src instead.

Checklist

• If my change required a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[julczka]

Thanks for fixing this!