fix Settings modal UI glitch with overlow in dark mode (#3330)

Summary:
This small tweak fixes the Setting modal UI glitch (white rectangle in the bottom right) related to the modal content overflow setting.

## Changelog

* fix Settings modal UI glitch with content overflow setting in dark mode

Pull Request resolved: https://github.com/facebook/flipper/pull/3330

Test Plan:
The change has been testes by running the desktop Flipper app locally from source.

## Preview (before & after)

#### No scrollbar

<img width="390" alt="Screenshot 2022-01-23 at 15 45 47" align="left" src="https://user-images.githubusercontent.com/719641/150684278-b09031bd-9b8c-4c9c-9ac2-2dba3c1af63d.png">
<img width="390" alt="Screenshot 2022-01-23 at 15 45 39" src="https://user-images.githubusercontent.com/719641/150684279-151b83d6-c819-45e0-a662-3d294f4f6a77.png">

#### With scrollbar
<img width="390" alt="Screenshot 2022-01-23 at 15 51 40" align="left" src="https://user-images.githubusercontent.com/719641/150684331-a8f27623-9a2b-4ffe-b4c1-3fb06a6362a2.png">
<img width="390" alt="Screenshot 2022-01-23 at 15 51 47" src="https://user-images.githubusercontent.com/719641/150684330-57434f21-0d4d-4ef6-be41-fdd88d082d1b.png">

Reviewed By: aigoncharov

Differential Revision: D33738647

Pulled By: lblasa

fbshipit-source-id: f5a170950271302241a29d4382c481f2ce8bf4ec

desktop/flipper-ui-core/src/chrome/SettingsSheet.tsx

@@ -98,7 +98,7 @@ class SettingsSheet extends Component<Props, State> {
         title="Settings"
         footer={footer}
         bodyStyle={{
-          overflow: 'scroll',
+          overflow: 'auto',
           maxHeight: 'calc(100vh - 250px)',
         }}>
         {contents}