[Unified Histogram] Fix metric chart width from 90$ to 90%

[davismcphee]

Summary

This PR fixes a minor typo from #187981 where the metric chart width is being set to 90$ instead of 90%. Now it's the correct size and doesn't cost 90$ 🙂

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Flaky Test Runner was used on any tests changed
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[davismcphee]

/ci

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 083b6f9

Metrics [docs]

✅ unchanged

cc @davismcphee

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[jughosta]

Nice! 90$ less!

[stratoula]

This PR got all wrong from the beginning 🤦 Thanx Davis ❤️