fix import path of intersection-observer dependency in sentinel

[seanstrom]

Description and Motivation

There seems to be a runtime error due to importing the IntersectionObserver component incorrectly. This leads to the List component hitting a "component is undefined" error in the browser.

Here's a link to a codesandbox demonstrating the error:
https://codesandbox.io/s/kind-bhaskara-4eznk?file=/src/App.js

How has this been tested?

• I built, linked, and bundled a local copy of the project into a client app.
• I ran the local test suite and storybook components server.

Note:

It seems like the test suite mocks the IntersectionObserver component dependency, so that may effect its passing results.

Type of change

• 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)

Checklist:

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my changes
• All new and existing tests passed

[danez]

I guess this is the same issue as with the tests here #124