Broken link in Popper documentation

[mitchellstern]

The link to the Popper modifiers documentation on https://material-ui.com/api/popper/ is broken.

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The link in the modifiers prop description points to https://github.com/FezVrasta/popper.js/blob/master/docs/_includes/popper-documentation.md#modifiers--object which is a broken link.

Expected Behavior 🤔

The link should be updated to point to the new location. Because Material-UI is using Popper.js v1.16, I believe this should be one of the following:

• https://github.com/popperjs/popper-core/blob/master/docs/src/pages/docs/v1/index.mdx#modifiers--object (preferred)
• https://github.com/popperjs/popper-core/blob/v1.16.0/docs/_includes/popper-documentation.md#modifiers--object (specific to v1.16)

Steps to Reproduce 🕹

Steps:

1. Navigate to https://material-ui.com/api/popper/#props.
2. Click the link with text 'read the modifiers documentation' in the modifiers prop description.