PropTypes.string.describe is not a function

Following the README instructions to install and use this module results in the following error

```
TypeError: react_desc__WEBPACK_IMPORTED_MODULE_2__.PropTypes.string.describe is not a function
```

# Reproduction steps

Repro repo: https://github.com/lokimckay-references/react-desc-26

1. `npx create-react-app myapp --use-npm`
2. `cd myapp`
3. `npm install react-desc`
4. Create Anchor.js file and copy + paste code snippet found in README
5. In `App.js`, attempt to access the JSON output as described in readme
```
import { AnchorWithSchema } from './Anchor';
const documentation = AnchorWithSchema.toJSON();
```

# Actual result


# Expected result

JSON describing Anchor component PropTypes as per README

## Versions


```
"dependencies": {
"@testing-library/jest-dom": "^5.14.1",
"@testing-library/react": "^11.2.7",
"@testing-library/user-event": "^12.8.3",
"react": "^17.0.2",
"react-desc": "^4.1.3",
"react-dom": "^17.0.2",
"react-scripts": "4.0.3",
"web-vitals": "^1.1.2"
},
```