Charts: Fixing theme storybook story

[grzegorz-cp]

Fixes n/a

Proposed changes:

• After introducing multi-series support for line charts I didn't update theme storybook

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• navigate to the directory and run storybook: cd projects/js-packages/charts and pnpm run storybook
• verify that the theme storybook is working

[github-actions]

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