fix: be explicit about Treeview group indicator box model

[lazd]

Description

Be explicit about Treeview group indicator box model.

Fixes #467

How and where has this been tested?

• How this was tested: Open treeview docs, add rule: * { box-sizing: border-box; } and make sure the indicator was still there
• Browser(s) and OS(s) this was tested with: Chrome on macOS

Screenshots

To-do list

• If my change impacts other components, I have tested to make sure they don't break.
• If my change impacts documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.

• This pull request is ready to merge.

[jianliao]

https://spectrum-visual-regression.ci.corp.adobe.com/job/spectrum-css-vr-test-asset/162/artifact/spectrum-css/backstop_data/html_report/index.html